UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


THE  GIFT  OF 

MAY  TREAT  MORRISON 

IN  MEMORY  OF 

ALEXANDER  F  MORRISON 


THE   ELEMENTS    OF   ASTRONOMY 


tj 

55    * 


THE  ELEMENTS  OF  ASTRONOMY 


BY 


SIR  ROBERT   BALL,  LL.D.,  F.R.S. 

LOWNDEAN    PROFESSOR   OF    ASTRONOMY    AND    GEOMETRY 

IX   THE   UNIVERSITY   OF  CAMBRIDGE 
AND   FORMERLY  ROYAL  ASTRONOMER  OF   IRELAND 


Ntfo 
THE   MACMILLAN    COMPANY 

LONDON:  MACMILLAN  &  CO.,  LTD. 
1900 

JLll  rights  reserved 


COPYRIGHT,  1900, 
BY  THE  MACMILLAN  COMPANY. 


J.  S.  Cuihing  &  Co.  -  Berwick  fc  Smith 
Norwood  Man.  U.S.A. 


Scie««S 


PREFACE 

FOK  the  pictures  with  which  this  little  work  is 
illustrated  I  have  thankfully  to  acknowledge  my 
obligation  as  follows.  To  the  Astronomer  Royal 
for  a  photograph  of  the  Sun.  To  Professor  MICHIE 
SMITH  and  Professor  SCHAEBERLE  for  their  photo- 
graphs  showing  features  in  a  Total  Eclipse.  To 
Professor  FAUTH  for  his  drawings  of  Jupiter.  To 
Mr.  W.  E.  WILSON,  F.R.S.,  for  his  photographs  of 
the  Dumb-bell  Nebula  and  of  the  cluster  in  Her- 
cules.  To  Professor  E.  E.  BARNARD  I  owe  the 
drawing  of  Saturn,  the  photograph  of  Swift's  Comet, 
1892,  the  photograph  showing  the  clusters  in  Per- 
seus,  and  the  photograph  of  Holmes'  Comet  and  the 
great  Nebula  in  Andromeda. 

I  must  also  acknowledge  the  kindness  with  which 
the  Royal  Astronomical  Society  placed  these  photo- 
graphs taken  from  their  beautiful  series  at  my 


ROBERT  S.  BALL. 
CAMBRIDGE, 

1900. 


429004 


CONTENTS 

PAUL 

INTRODUCTION 1 

CHAPTER   I 

The  Diurnal  Motion    .  5 

CHAPTER  II 
The  Sun 24 

CHAPTER   III 

The  Apparent  Motion  of  the  Sun 47 

CHAPTER   IV 
The  Moon 60 

CHAPTER  V 

Gravitation 82 

CHAPTER  VI 

Mercury  and  Venus 98 

CHAPTER  VII 
Mars 104 

CHAPTER  VIII 
The  Asteroids .112 

CHAPTER  IX 
Jupiter 119 

vii 


Contents 
CHAPTER   X 
CHAPTER   XT 


Saturn.        .        .........     13° 


Uranus  and  Neptune   ........ 

CHAPTER  Xn 
Comets  ..........  146 

CHAPTER  XTTT 
Shooting  Stars  .........  152 

CHAPTER  XTV 
Stars  and  Nebulae  ........  I57 

CHAPTER   XV 

Causes  affecting  the  Apparent  Places  of  the  Stars      .         .     17:2 

INDEX.  .....     181 


PLATES 

Saturn,  July  2,  1894 Frontispiece 

The  Sun To  face  page  26 

Total  Solar  Eclipse,  1893     ....  «  "  40 

Total  Solar  Eclipse,  1898     ....  "  «  4L> 

The  Full  Moon,  Age  14  days.  8  hours          .  '•  "  70 

Jupiter,  1897 '•  «  120 

Swift's  Comet,  1892 '•  ••  148 

Cluster  (M.  13)  Herculis     ....  ••  159 

Milky  Way  near  Cluster  in  Perseus    .        .  ••  '  -  160 

Dumb-bell  Nebula       .....  «  «  163 
The    Great    Nebula    in    Andromeda    and 

Holmes'  Comet    ...  "  ••  164 


THE   ELEMENTS    OF   ASTRONOMY 


INTRODUCTION 


IT  is  impossible  for  us  now  to  know  what  were  the 
earliest  beginnings  of  astronomical  knowledge.  Many  of 
the  remarkable  discoveries,  such,  for  instance,  as  the 
recognition  of  the  principal  planets,  were  made  in  pre- 
historic times.  The  very  earliest  allusion  which  histori- 
ans have  been  able  to  discover  refers  to  them  as  objects 
which  were  already  well  known.  It  is,  however,  reasonable 
to  suppose  that  the  first  of  all  celestial  problems  which 
occupied  intelligent  man  must  have  been  the  rising  and 
the  setting  of  the  Sun.  So  long  as  the  Earth  was  believed 
to  consist  of  an  indefinitely  extended  plane,  it  was  hard 
to  realise  that  the  Sun  which  disappeared  in  the  West  one 
evening  was  indeed  the  self-same  object  as  that  which  rose 
in  the  East  on  the  following  morning.  Probably  this  fact 
alone  led  the  earliest  philosophers  to  the  conclusion  that 
however  the  apparent  evidence  of  the  senses  might  lead 
to  an  opposite  conclusion,  it  was  nevertheless  certain  that 
the  Earth  could  not  be  an  indefinitely  extended  plane,  but 
that  it  must  be  a  detached  and  isolated  body  so  that  the 
Sun  was  able  to  dip  down  under  it,  as  it  were,  in  the 
coxirse  of  its  nightly  journey.  Once  this  step  had  been 
taken  arguments  were  easily  forthcoming  to  shew  that  the 
Earth  was  of  globular  form.  The  symmetry  of  the  spheri- 
cal surface  would  naturally  appeal  to  the  taste  of  the 

K  1 


2  Astronomy 

early  geometers,  and  when  they  saw  that  the  Sun  and 
the  Moon  were  also  spherical,  then  the  doctrine  that  the 
Earth  is  indeed  a  mighty  sphere  became  an  accepted 
contribution  to  knowledge. 

The  earliest  observation  also  associates  the  changes  of 
the  Seasons  with  certain  alterations  in  the  apparent  posi- 
tion of  the  Sun.  It  was  obvious  that  the  Sun  remained 
low  down  in  the  heavens  during  the  winter,  even  at  noon. 
In  summer,  on  the  other  hand,  the  Sun  ascended  high  in 
the  heavens.  Thus  it  was  clear  that  the  Sun  was  not 
a  fixed  point  on  the  celestial  sphere,  so  that  even  if  the 
phenomenon  of  rising  and  setting  was  produced  by  the 
revolution  of  the  celestial  sphere,  still  some  independent 
movement  had  to  be  attributed  to  the  Sun.  The  acuteness 
of  the  early  observers  led  them  to  distinguish  the  different 
stars  in  the  sky.  They  saw  that  these  stars  were  arranged 
in  certain  definite  groups,  and  they  noticed  the  remarkable 
fact  that  the  stars  belonging  to  these  groups  retained  their 
celestial  positions  as  permanently  as  the  Alps  or  other 
mountains  on  the  earth  remained  fixed  in  their  terrestrial 
places.  It  was  natural  to  watch  how  these  constellations 
came  into  visibility  as  soon  as  the  twilight  of  evening 
had  subsided.  And  then  a  little  attention  revealed  to  the 
early  astronomers  the  interesting  fact  that  the  constella- 
tions which  came  into  view  at  the  sunsets  in  the  West 
were  not  the  same  throughout  the  year.  They  saw  that 
these  constellations  changed  with  the  seasons.  At  last 
it  was  noticed  that  when  a  year  had  elapsed,  the  same 
constellations  returned  to  their  original  positions.  Take, 
for  instance,  one  of  the  most  remarkable  groups,  the 
constellation  of  Taurus.  At  certain  seasons  the  stars  of 
this  famous  group  were  found  to  be  situated  in  the  West 
as  soon  as  the  light  of  the  departing  Sun  had  sufficiently 
faded  to  allow  them  to  become  visible.  But  after  a  few 
weeks  these  stars  ceased  to  be  seen,  they  had  passed 


Introduction  3 

nearer  and  nearer  to  the  West  until  at  last  by  the  time  the 
sunlight  had  declined  the  stars  in  Taurus  had  passed 
below  the  horizon.  Not  for  another  year  was  this  con- 
stellation to  be  seen  in  the  same  position,  but  then  all 
the  phenomena  were  precisely  repeated.  A  little  further 
consideration  pointed  out  what  the  cause  of  these  changes 
must  be.  It  was  no  movement  of  the  stars  themselves. 
It  was  obvious  that  the  changes  must  be  attributed  to  the 
movements  of  the  Sun.  As  the  Sun  advanced  in  its 
course  it  came  near  Taurus,  and  then  the  stars  of  that 
constellation  set  with  the  Sun.  The  same  was  true  of 
many  other  constellations  and  hence  it  became  manifest 
that  the  stars  were  strewn  all  round  the  celestial  sphere, 
and  that  the  Sun  apparently  performed  an  annual  revolu- 
tion in  a  track  amongst  the  stars.  This  track  was  carefully 
marked  out,  and  the  route  which  it  follows,  laid  down  by 
the  sagacity  of  these  early  observers,  is  the  circle  which  we 
now  call  the  ecliptic. 

So  long  as  the  Earth  appeared  to  be  a  body  of  vast 
magnitude  with  regard  to  the  stars,  and  at  a  time  when 
the  stars  and  other  celestial  bodies  were  believed  to  be  at 
no  very  great  distance  from  the  Earth,  it  seemed  natural 
to  suppose  that  the  fundamental  phenomena  of  rising  and 
setting  were  caused  by  the  rotation  of  the  whole  celestial 
sphere,  bearing  with  it  the  stars,  the  sun  and  the  moon, 
and  all  the  other  celestial  bodies.  But  when  it  began  to 
lie  realised  that  the  dimensions  of  the  Earth  were  after  all 
but  small  in  comparison  with  the  distances  at  which  the 
heavenly  bodies  were  placed,  then  suspicions  arose  that 
possibly  this  apparent  movement  of  rising  and  setting  must 
be  accounted  for  in  another  way.  Once  it  had  been  shewn 
by  geometers  that  all  the  phenomena  which  were  actually 
observed  could  be  explained  either  by  the  rotation  of  the 
celestial  sphere  in  one  direction,  or  by  the  rotation  of  the 
Earth  itself  in  the  opposite  direction,  there  could  no  longer 


4  Astronomy 

l)e  much  doubt  as  to  the  true  explanation.  It  was 
obviously  more  rational  to  suppose  that  the  Earth  turned 
round  once  every  twenty -four  hours  than  that  the  stu- 
pendous fabric  of  the  celestial  sphere  with  the  heavenly 
bodies  upon  it  could  accomplish  a  rotation  in  the  opposite 
direction  in  the  same  time. 

As  the  Sun  performed  its  annual  movements  along  the 
ecliptic  which  runs  through  the  signs  of  the  Zodiac,  it  was 
for  ages  supposed  that  the  great  luminary  did  therefore 
actually  make  an  annual  revolution  around  the  Earth. 
Here  again,  however,  it  was  shewn  that  the  apparent 
movement  was  really  quite  different  from  the  real  one. 
Copernicus  (1473-1543)  pointed  out  how  the  phenomena 
as  to  the  seasonal  change  of  the  Sun's  altitude  in  the  heav- 
ens, and  as  to  the  passage  of  the  Sun  through  the 
various  constellations  which  mark  out  the  signs  of  the 
Zodiac,  could  all  be  accounted  for  in  a  much  simpler 
manner.  He  made  the  bold  supposition  that  the  Earth, 
besides  its  rotation  around  its  axis,  also  performs  a  move- 
ment of  revolution  around  the  Sun,  accomplishing  this 
revolution  in  the  course  of  a  year. 

Thus  was  our  knowledge  of  the  celestial  movements 
advanced  to  the  stage  from  which  modern  Astronomy  takes 
its  departure. 


CHAPTER  I 
THE  DITJENAL  MOTION 

§  1.  Shape  and  Size  of  the  Earth.  We  learn  in  our 
geography  books  the  well-known  fact  which  demonstrates 
that  the  Earth  is  not  the  flat  surface  which  a  first  glance 
would  seem  to  indicate,  but  that  it  is  of  a  more  or  less 
spherical  form.  More  precisely  we  describe  the  figure 
of  the  Earth  as  produced  by  the  revolution  of  an  ellipse 
around  its  shorter  axis.  According  to  the  best  deter- 
minations the  equatorial  semi-diameter  of  the  ellipse  is 
20,926,000  feet  and  the  length  of  the  polar  semi-diameter 
is  20,855,000  feet,  and  from  these  figures  we  easily  deduce 
that  the  ellipticity,  by  which  we  mean  the  ratio  which 
the  difference  between  the  two  axes  of  the  ellipse  bears 
to  the  larger,  is  1/295. 

§  2.  Atmospheric  Refraction.  The  Earth  is  surrounded 
by  an  atmosphere  with  a  density  greatest  at  the  surface 
of  the  Earth  and  steadily  diminishing  until  the  upper 
limit  of  the  atmosphere  is  reached.  The  actual  height  to 
which  the  atmosphere  extends  cannot  be  stated  precisely. 
It  has  been  found  that  shooting-stars  are  sometimes  seen  at 
an  altitude  of  more  than  two  hundred  miles,  and  since  these 
bodies  are  only  rendered  visible  by  the  resistance  which 
our  atmosphere  offers  to  their  motion  we  conclude  that  the 


which 


a  star  entering 

ance  with  these  laws  bent  down  thr^   ^'    }  "  m  a<3C°rd- 

towards  the  centre  °Ugh  a  Vei7  sma11  angle 

of  the  Earth.     We 

may    suppose    the 

atmosphere    to    be 

composed  of  a  very 

large    number     of 

successive  layers,  or 

strata,    lying    one 

below  the  other  and 

increasing  in  den- 

sity   towards    the 

Earth's  surface. 

When     the     ray 


from  layer  to  layer  1  7/7     am°Unt 

e  atmosphere     ThP    I        °WS  a  Curved 
eser 


The  Diurnal  Motion  7 

each  of  the  successive  strata  through  which  it  passes.  But 
at  any  distance  from  the  zenith,  for  so  the  point  vertically 
overhead  is  termed,  down  to  the  horizon,  the  refraction 
gradually  increases.  Thus,  for  instance,  at  an  apparent 
/enith  distance  of  45°  the  effect  of  refraction  is  to  make  a 
star  appear  58"-2  higher  than  it  ought  to  appear  if  the 
air  were  absent.  Towards  the  horizon  refraction  increases, 
and  becomes  34'  when  a  star  is  actually  at  the  horizon, 
but,  generally  speaking,  refraction  may  be  taken  as  pro- 
portional to  the  tangent  of  the  zenith  distance  for  mod- 
erate distances  from  the  zenith. 

§  3.  The  Celestial  Sphere.  The  various  celestial 
bodies  are  conventionally  supposed  by  the  astronomer  to  be 
on  the  surface  of  a  sphere  which  we  call  the  celestial  sphere. 
Of  course  I  need  hardly  say  that  the  stars  are  at  very 
varied  distances  from  the  Earth,  but  nevertheless  the 
appearance  of  the  heavens  can  be  represented  on  a  globe 
of  which  the  observer  is  supposed  to  occupy  the  centre. 
With  regard  to  the  ordinary  stars,  their  distances  are  so 
enormous  as  compared  with  the  dimensions  of  the  Earth, 
that  the  size  of  the  latter  may  be  absolutely  neglected,  and 
observers  in  all  parts  of  the  Earth  may  be  considered 
equally  as  occupying  the  centre  of  the  celestial  sphere. 

"\Vheii  dealing  with  the  members  of  the  solar  system, 
whose  distances  though  vast  are  not  so  enormous  as  to 
justify  us  in  considering  the  Earth  as  a  mere  point  at 
the  centre,  it  is  often  necessary  to  take  into  account  the 
position  of  the  observer  on  the  surface  of  the  Earth. 

§  4.  The  Constellations.  The  majority  of  the  objects 
visible  in  the  sky  are  known  as  the  fixed  stars  ;  there  are 
only  five  planets  which  are  conspicuously  visible  to  the 
unaided  eye,  namely,  Mercury,  Venus,  Mars,  Jupiter  and 
Saturn.  The  fixed  stars  have  been  classified  according 
to  their  degrees  of  brightness.  The  brightest  stars  are 
those  of  the  first  magnitude,  such  as  Sirius,  Arctums, 


8  Astronomy 

Vega  and  Capella.  The  next  order  of  magnitude  may 
be  illustrated  by  the  stars  which  form  the  well-known 
constellation  of  the  Great  Bear.  The  stars  below  those 
again  would  be  the  third  magnitude,  and  so  down  to  the 
very  faintest  stars  which  could  be  seen  with  the  most 
powerful  telescope.  Of  the  first  magnitude  stars  the 
number  is  nineteen,  of  the  sixth  there  are  nearly  five 
thousand,  while  of  the  ninth  there  are  about  a  quarter 
of  a  million.  A  star  of  the  first  magnitude  is  about  a 
hundred  times  as  bright  as  one  of  the  sixth.  Stars  of  the 
fifth  magnitude  are  faint  to  the  unaided  eye,  while  those 
of  the  seventh  can  but  rarely  be  perceived  without  a  tele- 
scope. The  numbers  of  the  stars  increase  enormously  as 
we  include  the  fainter  objects.  Argelander's  famous 
chart  of  the  northern  hemisphere  contains  324,188  stars. 
All  stars  of  the  first  nine  magnitudes  were  included  in 
this  list,  and  a  considerable  number  also  between  the 
ninth  and  tenth  magnitudes.  The  total  number  of  stars 
now  known  must  be  reckoned  by  scores  of  millions. 

The  prodigious  multitude  of  minute  stars  is  well  shewn 
in  the  Milky  Way,  that  broad  band  of  light  across  the 
heavens.  One  of  the  earliest  results  of  the  application  of 
the  telescope  to  celestial  spaces  was  to  prove  that  the 
Milky  Way  was  composed  of  myriads  of  stars,  generally 
speaking  too  minute  to  be  discernible  with  the  unaided  eye, 
but  producing  by  their  clustering  myriads  the  luminous 
effect  which  is  so  well  known.  A  photographic  plate 
exposed  in  a  properly  mounted  camera  for  a  few  hours  will 
record  the  impression  of  uncounted  thousands  of  stars  in 
almost  any  part  of  the  Milky  Way.  In  certain  places 
these  stars  accumulate  in  such  abundance  that  it  seems 
almost  impossible  to  discriminate,  in  the  coruscating  mass, 
the  individual  stellar  points  which  contribute  to  it.  Some- 
times, on  the  other  hand,  we  are  astonished  to  see  vacant 
tracts  in  which  few  stars  are  to  be  found.  From  the 


The  Diurnal  Motion  9 

earliest  times  it  was  found  necessary  for  convenience  in 
studying  the  heavens  to  divide  the  stellar  regions  into 
groups,  which  are  known  as  constellations.  This  early 
method,  which  has  survived  to  the  present  day,  supposes 
the  surface  of  the  celestial  sphere  to  be  covered  with 
imaginary  representations  of  human  figures  and  other 
objects.  By  some  grotesque  associations  the  bright  stars 
in  the  sky  are  made  to  indicate  the  forms  of  the  objects. 
Whatever  may  be  said  as  to  the  art  or  the  science  of  this 
scheme  it,  at  all  events,  provides  us  with  the  convenience 
of  a  special  name  for  each  part  of  the  sky,  the  stars  in 
each  region  being  termed  a  constellation.  We  must  refer 
to  an  atlas  of  the  stars  for  a  description  of  these  constel- 
lations, and  it  will  be  necessary  for  the  student  by  the  aid 
of  such  an  atlas  to  make  himself  familiar  with  the  posi- 
tions of  the  leading  groups. 

§  5.  The  Diurnal  Motion  of  the  Sphere.  The  diurnal 
motion  of  the  stars,  in  which  of  course  the  Sun,  the 
Moon  and  planets  also  participate,  has  now  to  be  considered. 
Take  some  particular  constellation,  and  for  this  purpose 
the  constellation  known  to  astronomers  as  Ursa  Major  and 
to  many  people  in  this  country 
as  The  Plough  is  very  conven- 
ient  (Fig.  2).  It  is  convenient 
because  whenever  the  sky  is 
clear  this  particular  group  will  \ 

be  found  above  the  horizon.    The  \ 

first  observation  to  be  made  is  to  \ 

note  the  position  of  Ursa  Major  \ 

with  reference  to  the  surrounding  \« 

objects.    The  observation  is  to  be  ,t    .,      ., 

repeated  a  few  hours  later.     A     .,  .y 

very  remarkable  change  will  have .         u«a  Major 
taken  place.    It  will  be  seen  that       pjg  2.    The  Pole  and 
the     whole     constellation     has  the  Pole  Star. 


10  Astronomy 

shifted  bodily.  The  apparent  angular  distances  of  the  stars 
in  the  constellation  from  each  other  have  not  indeed  altered, 
but  the  whole  constellation  has  been  displaced  relatively 
to  the  terrestrial  objects.  A  like  observation  may  be 
made  with  any  other  constellation  that  is  visible  and  in 
an  hour  or  two  the  changes  in  its  position  will  be  obvious. 

The  learner  must  specially  make  himself  acquainted 
with  that  most  important  star  in  the  northern  hemi- 
sphere which  is  known  as  '  the  Pole  Star.'  It  is  easily 
indicated  by  the  two  leading  stars  in  the  Great  Bear  which 
are  called  '  the  Pointers/  because  the  straight  line  joining 
them  points  very  nearly  to  the  Pole.  At  different  hours  of 
the  night,  or  even  at  different  seasons  of  the  year,  the  Pole 
Star  will  ahvays  be  seen  in  the  northern  sky  at  what  is 
nearly  the  same  elevation  above  the  horizon.  The  fixity 
of  the  Pole  Star  appears  in  marked  contrast  to  the  never- 
ending  changes  in  the  position  of  the  constellations.  TVe 
do  not  indeed  say  that  the  Pole  Star  is  absolutely  fixed, 
but  the  amount  of  its  movement  is  quite  insensible  in  com- 
parison with  the  movements  of  the  other  constellations. 

It  would  seem  indeed  as  if  the  celestial  sphere  con- 
taining all  the  constellations  was  actually  revolving  about 
an  axis  which  passed  through  the  Earth's  centre  and  which 
also  passed,  I  cannot  say  through  the  Pole  Star  but  quite 
near  to  it,  piercing  the  celestial  sphere  in  points  which  are 
called  the  North  and  South  Poles.  This  movement  of  the 
constellations,  by  which  each  of  them  appears  to  complete 
a  circuit  of  the  celestial  sphere  once  a  day,  is  called  the 
diurnal  motion.  In  this  motion  the  relative  positions  of 
the  stars  are  unaltered  and  each  star  maintains  its  distance 
from  the  pole  unchanged  except  for  the  small  disturbance 
caused  by  the  atmospheric  refraction,  as  explained  above, 
the  amount  of  which  varies  as  the  star  changes  its  position. 

§  6.  Circles  of  the  Sphere.  It  can  easily  be  shewn 
that  if  a  point  on  a  sphere  rotates  so  as  to  be  always  at  the 


The  Diurnal  Motion  11 

same  distance  from  a  fixed  point  on  the  sphere  its  path  will 
be  a  circle.  For,  if  P  be  the 
fixed  point  and  C  the  centre 
of  the  sphere  and  if  S  be  any 
position  of  the  moving  point, 
then  the  arc  PS  and  conse- 
quently the  angle  SCP  are 
constant.  Let  fall  /SJVperpen- 
dicular  to  CP.  Then  in  the 
triangle  SCN,  the  angle  SCX 
is  constant,  the  angle  /SLVC'  is 
a  right  angle,  and  the  side  CS  Fig  3>  Great^  Small  Circles. 
is  constant,  being  the  radius 

of  the  sphere ;  hence  the  side  NS  is  constant  in  length. 
Also,  CNis  constant  and  therefore  J\ris  a  fixed  point. 

Inasmuch  as  *S'ATC  is  a  right  angle  it  follows  from  the 
fifth  proposition  of  the  eleventh  book  of  Euclid  that  all 
successive  positions  of  JV/S  lie  in  the  same  plane.  Hence 
the  path  of  S  must  be  some  plane  curve,  and  since  NS  is 
constant  in  length  and  NSL  fixed  point,  it  must  be  a  circle. 

Thus  we  see  that  every  star  will  appear  to  describe  a 
circle,  and  the  planes  of  all  these  circles  will  be  parallel  to 
each  other,  for  the  same  line  (viz.  CP,  the  polar  axis)  is 
perpendicular  to  them  all.  Circles  such  as  those  described 
by  the  stars  *S'  and  S'  (Fig.  3)  whose  planes  do  not  pass 
through  the  centre  of  the  sphere  are  called  small  circles, 
their  radii,  as  AT.S',  JWS',  being  less  than  that  of  the  sphere. 
Any  circle  on  the  sphere  whose  plane  passes  through  the 
centre  is  called  a  great  circle,  and  it  is  clear  from  the 
figure  that  if  the  angular  distance  of  a  star  from  the  pole 
be  exactly  a  right  angle  (or  90°)  the  plane  of  its  diurnal 
circle  will  pass  through  the  centre  and  hence  its  track  will 
be  a  great  circle.  This  particular  great  circle  which  is  every- 
where 90°  from  the  pole  is  a  very  important  one  in  astronomy. 
It  is  called  the  celestial  equator  or  simply  the  equator. 


12 


Astronomy 


We  may  therefore  state  generally  that  the  apparent 
diurnal  motions  of  the  stars,  and  here  we  must  include  also 
the  Sun,  the  Moon  and  the  planets,  are  performed  in  small 
circles  of  the  celestial  sphere,  and  all  these  small  circles  lie 
in  a  system  of  planes  parallel  to  the  celestial  equator.  It 
is  to  be  noted  that  so  far  as  the  stars  are  concerned  each 
one  moves  round  in  its  circle  with  uniform  velocity  and 
each  star  also  requires  the  same  time  for  the  completion  of 
its  circuit,  that  time  being  found  by  observation  to  be 
23  hrs.  56  mins.  4  sees.  The  times  required  by  the  Sun, 
the  Moon  and  the  planets  vary. 

There  are  other  circles  on  the  celestial  sphere  which  it 
is  necessary  for  the  student 
to  understand.  A  line  hung 
vertically  with  a  weight  at  the 
end  determines  a  direction 
which  pointing  upwards  indi- 
cates the  point  of  the  sphere  al- 
ready referred  to  as  the  zenith 
and  pointing  downwards  indi- 
cates the  point  on  the  celestial 
sphere  known  as  the  nadir.  A 
plane  perpendicular  to  this 
line  through  the  observer's  eye 
cuts  the  celestial  sphere  in  the 
great  circle  which  we  call  the 
Jionzon.  If  a  great  circle  be 
drawn  through  the  pole  of  the 


Fig.  4.     The  Celestial  Sphere. 


heavens  and  the  zenith  and  be  continued  round,  it  would 
pass  also  through  the  nadir.  This  circle  is  called  the 
meridian  of  the  place  of  observation.  Another  important 
circle,  as  I  have  already  pointed  out,  is  that  which  is  at 
every  point  90°  distant  from  the  Pole.  This  great  circle 
is  spoken  of  as  the  equator. 

§  7.   The  Sidereal  Day.      The  period  of  revolution  of 


TJie  Diurnal  Motion  13 

each  of  the  stars  in  this  diurnal  motion  being  the  same,  the 
result  is  that  the  whole  sidereal  system  revolves  as  if  all  the 
bodies  were  in  one  piece.  The  time  taken  for  this  revolution 
is  constant.  It  furnishes  therefore  a  most  important  natural 
unit.  Its  length  measured  in  ordinary  solar  time  is  23  hrs. 
56  mins.  4  sees,  and  this  is  the  period  known  to  astronomers 
as  the  sidereal  day.  Like  our  ordinary  day  the  sidereal  day 
is  divided  into  24  (sidereal)  hours,  each  hour  into  60  min- 
utes and  each  minute  into  60  seconds.  The  standard  clock 
in  the  Observatory  is  regulated  to  keep  this  time,  sidereal 
time  as  it  is  called.  The  ordinary  time  convenient  for 
civil  purposes,  and  which  is  known  as  mean  solar  time,  has 
a  day  nearly  four  minutes  longer  than  the  sidereal  day. 

§  8.  The  Rotation  of  the  Earth.  It  would  of  course  be 
possible  to  account  for  this  diurnal  rotation  of  the  heavens 
so  far  as  the  mere  geometrical  phenomena  are  concerned, 
by  supposing  that  the  whole  celestial  sphere  did  actually 
turn  round  as  it  appears  to  do.  We  must  however  always 
be  prepared  in  the  study  of  astronomy  to  distinguish 
between  apparent  motions  and  real  motions.  A  little 
consideration  will  shew  that  all  the  phenomena  of  the 
diurnal  movement  can  be  accounted  for  by  the  supposition 
that  the  celestial  sphere  remains  at  rest,  but  that  the 
Earth  at  the  centre  is  rotating  uniformly  in  the  opposite 
direction,  and  accomplishing  each  complete  rotation  round 
its  axis  in  the  period  of  23  hrs.  56  mins.  4  sees.  Once  the 
issue  has  been  placed  in  this  way  it  is  impossible  to  hesitate 
as  to  whether  the  phenomenon  should  be  explained  by  the 
rotation  of  the  celestial  sphere  or  by  the  rotation  of  the 
Earth.  When  it  was  understood  that  the  various  celestial 
bodies  were  situated  at  widely  different  distances  and  that 
some  of  those  distances  were  excessively  great,  it  soon 
became  plain  that  it  would  be  hardly  short  of  miraculous 
if  the  whole  system  revolved  around  the  Earth  in  such  a 
manner  that  every  star,  no  matter  what  its  distance,  should 


14  Astronomy 

take  precisely  the  same  time  to  complete  the  circuit. 
The  velocities  also  with  which  the  various  bodies  would 
have  to  be  endowed  in  order  to  accomplish  such  a  revolu- 
tion would  be  exceedingly  great.  Take,  for  instance,  the 
case  of  the  Sun.  It  would  be  preposterous  to  suppose  that 
this  body,  between  one  and  two  million  times  bigger  than 
the  Earth,  should  whirl  round  the  Earth  once  each  day  at 
a  distance  of  nearly  ninety-three  million  miles,  when  all 
the  observed  phenomena  could  be  equally  well  accounted 
for  by  the  much  simpler  supposition  that  the  Earth  is 
itself  in  rotation.  This  then  is  the  first  great  step  in 
our  comprehension  of  the  heavens.  It  is  to  learn  that 
this  diurnal  movement  is  merely  apparent  and  that  what 
actually  takes  place  is  a  rotation  of  the  Earth. 

§  9.  Fundamental  Observation.  The  fundamental  ob- 
servation in  the  Observatory  consists  in  determining  the 
place  of  a  celestial  body,  say,  for  instance,  a  star.  Let  it 
be  supposed  that  we  want  to  determine  the  position  of  the 
star  named  Vega.  To  indicate  its  place  on  the  celestial 
sphere  we  must  know  first  of  all  its  distance  from  the  Pole. 
That  is  to  say,  of  course,  the  number  of  'degrees  in  the 
distance  between  the  point  on  the  celestial  sphere  that  we 
call  the  Pole,  and  the  point 
which  is  occupied  by  the  star 
Vega.  This  is  one  of  the 
elements  in  the  determina- 
tion of  the  place  of  the  star, 
but  it  is  of  course  not  suffi- 
cient by  itself.  It  merely 
points  out  a  certain  small 
circle  on  the  sphere  (VA, 
Fig.  5),  every  point  of  this 
small  circle  being  at  the 

same  distance  from  the  Pole,       Fig.  6.     Riffht  Ascension  and 
and   the   observation    tells  Declination. 


The  Dluritcd  Motion  15 

us  that  the  star  Vega  is  somewhere  or  other  along  the  cir- 
cumference of  this  small  circle.  We  must  determine  the 
position  of  the  star  upon  the  small  circle  and  for  this 
purpose  we  require  to  mark  some  point  of  reference  upon  it. 
Let  us  take  some  star  as  a  standard  of  reference ;  let  us 
say,  for  instance,  the  bright  star  Sirius,  and  mark  its  place 
on  the  celestial  sphere  by  the  letter  S  and  now  draw  the 
great  circle  PS  through  the  Pole  and  Sirius.  This  will  cut 
the  small  circle  along  which  Vega  lies  in  a  certain  point 
that  we  may  call  A.  We  might  select  this  point  A  as 
the  standard  point  on  the  small  circle  and  the  arc  AV 
measured  from  A  up  to  the  position  V  of  Vega  would  give 
the  position  of  that  star.  It  is,  however,  not  convenient  to 
measure  arcs  along  a  small  circle,  so  instead  of  taking  that 
arc  we  shall  take  the  angle  which  the  arc  subtends  at  the 
Pole,  that  is  to  say  the  angle  between  the  two  great  circles, 
VP  and  SP.  We  can  thus  completely  determine  the  posi- 
tion of  this  particular  star  Vega.  We  want  to  know  first 
of  all  its  distance  from  the  Pole,  then  starting  from  the 
Pole  we  measure  the  given  distance  PA  011  the  standard 
great  circle  SP.  Through  this  a  small  circle  is  to  be 
drawn  with  the  Pole  as  centre  and  then  the  required 
angle  8PV  is  to  be  marked  off.  This  will  indicate  the 
position  of  Vega. 

It  will  thus  be  plain  that  for  the  determination  of  the 
position  of  an  object  on  the  celestial  sphere  two  quantities 
are  necessary,  the  distance  from  the  Pole  and  the  angle 
between  the  standard  great  circle  and  the  great  circle 
passing  through  the  object  and  the  Pole.  By  the  standard 
great  circle  we  mean  a  great  circle  passing  through  the 
Pole  and  some  standard  object.  Considering  that  the 
whole  celestial  sphere  is  in  apparent  rotation  accomplishing 
a  complete  turn  once  in  a  sidereal  day,  it  is  very  natural 
and  it  is  extremely  convenient  to  measure  the  angle 
l>rtwrcn  these  two  great  circles  not  in  the  way  in  which 


16  Astronomy 

angles  are  usually  measured,  by  degrees,  minutes  and 
seconds,  but  to  measure  it  as  a  matter  of  time,  in  hours, 
minutes  and  seconds. 

§  10.  The  Meridian  Circle.  We  can  now  describe  the 
most  fundamental  observation  of  an  observatory.  We 
shall  suppose  that  we  have  at  our  disposal  a  clock  which 
has  been  carefully  rated  to  keep  sidereal  time.  The  in- 
strument with  which  the  observation  is  made  is  the  meri- 
dian circle.  In  former  days  it  required  two  instruments, 
the  transit  instrument  and  the  mural  circle,  to  make  a 
complete  observation  of  this  sort.  The  meridian  circle, 
however,  which  is  used  in  a  modern  observatory  for  this 
purpose  is,  in  fact,  a  combination  of  the  two. 

The  transit  instmment  consists  of  a  telescope  which 
rotates  on  an  axis  fixed  at  right  angles  to  its  length.  The 
axis  is  carefully  adjusted  so  as  to  be  strictly  horizontal 
and  to  point  exactly  east  and  west.  When  placed  horizon- 
tally this  telescope  will  point  of  course  to  the  north ;  when 
raised  to  a  suitable  elevation  it  will  point  exactly  to  the 
Pole;  when  turned  vertically  upwards  it  points  to  the 
zenith.  In  fact,  an  eye  placed  at  that  telescope  as  it  is 
turned  around  its  axis  will  trace  out  the  meridian  on  the 
celestial  sphere,  and  to  give  precision  we  may  suppose  that 
a  single  fine  thread  has  been  stretched  across  the  eye-piece 
of  the  telescope  perpendicular  to  its  axis,  so  that  as  the 
telescope  is  moved  round,  this  thread,  projected  against 
the  sky,  will  always  coincide  with  the  meridian.  Such  is 
the  transit  instrument,  and  now  for  the  way  in  which  the 
transit  instrument  is  to  be  employed.  The  astronomer 
waits  till  Sirius  is  crossing  the  meridian.  This  is  the 
moment  when  Sirius,  rising  up  from  the  eastern  horizon, 
reaches  the  highest  point  of  its  daily  course  before  it 
begins  to  descend  towards  the  western  horizon.  The 
meridian  is  marked  to  the  observer  seated  at  the  telescope 
by  the  line  across  the  eye-piece,  and  he  notes  by  his  clock 


The  D:«rnal  Motion  17 

the  hour,  the  minute  and  the  second  at  which  Sirius 
passes  this  line,  and  records  the  time  thus  found. 

In  the  best  modern  observatories  an  instrument  called 
a  chronogmpli  is  used  by  which  the  time  of  such  an 
observation  can  be  registered  to  a  small  fraction  of  a 
second  by  merely  pressing  an  electric  key.  He  then  points 
the  telescope  to  that  part  of  the  meridian  at  which  Vega 
will  cross  and  waits  until  in  the  course  of  its  diurnal 
movement  Vega  comes  across  his  field  of  view,  and  then 
he  repeats  the  observation,  determining  once  again  with 
the  clock  the  hour,  minute  and  second  when  Vega  makes 
its  transit  across  the  line  in  the  eye-piece.  Subtracting 
the  time  in  the  former  observation  from  the  time  in  the 
latter,  he  learns  the  number  of  hours,  minutes  and  seconds 
in  the  angle  between  the  great  circle  drawn  from  the  Pole 
down  to  Sirius  and  the  great  circle  drawn  from  the  Pole 
down  to  Vega.  If  he  wishes  to  transform  the  measure  of  this 
angle  thus  expressed  in  time,  in  to  the  equivalent  expression 
in  degrees,  minutes  and  seconds,  then  he  can  easily  make 
the  change  by  simply  remembering  that  since  360°  (i.e.  the 
whole  circumference)  passes  the  meridian  in  24  hours, 
fifteen  degrees  of  arc  are  equivalent  to  one  hour  of  time. 
But  it  will  not  generally  be  necessary  to  make  this  change, 
since  astronomers  always  prefer  to  keep  these  angles  ex- 
pressed in  time  as  they  are  thus  much  more  convenient  for 
immediate  comparison  with  other  observations.  Thus  one 
element  of  the  place  of  Vega  has  been  obtained.  We  have 
now  to  learn  its  angular  distance  from  the  Pole. 

To  effect  this  determination  the  astronomer  employs  in 
all  modern  observatories  what  is  known  as  the  meridian 
circle.  This  consists  of  a  transit  instrument  provided  with 
a  circle  perpendicular  to  the  axis  of  rotation  and  concentric 
with  it,  the  circle  being  carefully  divided  into  degrees, 
minutes  and  seconds,  and  provision  is  made  by  the  help  of 
microscopes  to  enable  these  divisions  on  the  circle  to  be 


IS 

read  off  with  extreme  precision.  We  shall  suppose  that 
once  for  all  the  angular  distance  of  Sirius  from  the  Pole  has 
been  obtained,  and  we  shall  accordingly  employ  Sirius  to 
give  not  merely  the  first  element  of  the  position  of  Vega 
but  also  the  second.  When  the  observation  of  Sirius  is 
being  made  the  astronomer  places  it  centrally  in  the 
telescope  at  the  moment  it  is  passing  the  meridian.  To 
enable  this  to  be  done  with  precision  the  meridian  circle  is 
provided  with  a  second  line  in  its  eye-piece  at  right  angles 
to  that  already  described,  so  that  when  the  star  is  in  the 
field  of  view  the  observer,  by  means  of  a  slowly-moving 
screw,  can  so  carefully  adjust  the  telescope  that  the  star 
runs  along  the  line  when  in  the  act  of  transit.  He  will 
then  read  off  his  circle  and  mark  the  number  of  degrees, 
minutes  and  seconds  corresponding  to  the  point  immedi- 
ately under  the  index.  Next,  when  Vega  is  passing  the 
meridian  he  will  take  a  similar  observation,  he  will  place 
the  telescope  so  that  Vega  runs  exactly  along  the  horizontal 
wire  and  he  will  read  the  circle  and  record  the  degrees, 
minutes  and  seconds,  indicated  by  the  reading  microscope. 
He  will  now  subtract  the  reading  that  he  gets  for  Vega  from 
the  reading  that  he  previously  found  for  Sirius,  and  the 
difference  between  the  two  will  give  the  difference  between 
the  distance  of  Vega  from  the  Pole  and  the  distance  of  Sirius 
from  the  Pole.  I  have  supposed  that  the  observer  knows 
the  distance  of  Sirius  from  the  Pole  and  hence  he  is  able, 
by  a  simple  subtraction,  to  obtain  the  angular  distance  of 
Vega  from  the  Pole.  This  observation  can  be  repeated 
with  any  other  star,  or  it  can  be  repeated  with  any  planet 
or  with  the  Sun  or  with  the  Moon  or  a  comet.  In  other 
words,  having  taken  the  place  of  Sirius  as  a  standard  it 
will  be  possible  to  determine  with  all  desired  precision 
the  place  of  every  other  object  on  the  celestial  sphere. 

§  11.    The  Equinox.    Such  is  an  outline  of  the  process 
by  which  the  places  of  the  celestial  bodies  are  recorded. 


The  Diurnal  Motion  19 

There  are  a  multitude  of  other  details  to  be  attended  to. 
For  instance,  the  refraction  of  the  atmosphere,  as  I  have 
already  explained,  always  tends  to  make  a  star  appeal- 
higher  above  the  horizon  than  it  actually  is.  The  obser- 
vations have  therefore  to  be  cleared  from  the  effect  of 
refraction  in  order  to  exhibit  the  place  of  the  star  as  it 
would  have  been  had  there  been  no  such  disturbing  effects 
of  the  atmosphere.  Then  too  I  have  spoken  of  Sirius  as 
the  standard  point  from  which  all  the  other  measurements 
were  made.  But  any  other  star  might  equally  have  been 
chosen  as  a  standard,  or  indeed,  any  other  point  in  the 
heavens,  provided  it  was  possible  to  clearly  define  what  that 
point  was.  As  a  matter  of  fact  we  do  not  ultimately  use  any 
star  but  we  employ  instead  a  certain  point  in  the  heavens, 
a  point  called  the  equinoctial  point  or  simply  the  equinox, 
Avhich  is  the  intersection  of  the  ecliptic  and  the  equator. 

§  12.  To  determine  the  position  of  the  Equator.  The 
ecliptic  is  the  great  circle  of  the  sphere  in  which  the  Sun 
performs  its  apparent  annual  movement.  To  understand 
clearly  how  the  position  of  the  equinox  is  determined  it 
will  be  necessary  to  refer  to  the  next  chapter,  where  the 
apparent  motion  of  the  Sun  is  considered.  But  it  will 
be  convenient  to  point  out  here  how  the  position  of  the 
equator  is  ascertained. 

It  is  clear  that  each  star  in 
the  course  of  its  daily  revolu- 
tion will  twice  cross  the  great 
circle  of  the  meridian.  In  the 
case  of  most  stars  the  lower 
transit  will  take  place  when 
the  star  is  below  the  horizon 
and  therefore  invisible,  but 
stars  situated  not  very  far 
from  the  Pole  will  be  visible  at 
both  transits.  Such  stars  are  Fig.  <J.  Circumpolar  Star. 


20  Astronomy 

called  circu-mpolar.  Now  if  the  meridian  circle  be  pointed 
to  such  a  star  at  its  upper  transit  at  U,  Fig.  6,  and  again 
12  hours  later,  when  it  is  crossing  the  meridian  below  the 
Pole  at  L,  and  the  two  readings  of  the  circle  be  taken,  it 
is  quite  obvious  that  in  the  upper  position  the  star  must  be 
as  many  degrees  above  the  Pole  as  it  is  below  the  Pole  in 
the  lower  position.  If  therefore  we  add  the  two  readings 
together  and  divide  by  two  it  is  clear  that  we  shall  get  the 
reading  corresponding  to  the  position  of  the  instrument 
when  it  is  pointing  to  the  Pole.  Here  again  the  influence  of 
atmospheric  refraction  will  slightly  modify  the  result,  but 
its  effect  can  be  satisfactorily  calculated  and  allowed  for. 

In  this  way  we  can  direct  the  telescope  accurately  to 
the  Pole  and,  since  we  know  that  the  equator  is  everywhere 
90°  from  the  Pole,  we  have  only  to  turn  the  telescope 
through  a  right  angle  to  make  it  point  to  the  equator. 

§  13.  Right  Ascension.  The  moment  when  the  equi- 
nox is  passing  the  meridian  is  taken  as  the  beginning  of 
the  sidereal  day  and  we  set  our  sidereal  clock  so  that  at 
this  instant  it  shall  shew  0  hours,  0  minutes,  0  seconds. 
And  if  our  clock  is  going  correctly  then  when  this  equinox 
comes  round  to  the  meridian  again  the  clock  should  have 
gone  through  a  complete  twenty-four  hours  and  should 
again  exhibit  0  hours,  0  minutes,  0  seconds.  The  riyht 
ascension  of  a  star  is  the  time  Avhich  elapses  between  the 
instant  at  which  the  equinox  crosses  the  meridian  and  the 
time  of  transit  of  the  star.  The  clock  having  been  arranged 
so  as  to  shew  0  hours,  0  minutes,  0  seconds  when  the  first 
of  these  events  takes  place,  it  is  clear  that  the  time  indi- 
cated by  the  clock  at  the  time  of  transit  of  any  star  is  its 
right  ascension.  We  are  now  able  to  see  how  convenient 
the  sidereal  clock  is  for  the  purposes  of  the  astronomer. 
Each  object  in  the  heavens  has  its  own  right  ascension 
and  we  see  that  its  right  ascension  indicates  the  sidereal 
time  at  which  this  particular  object  will  be  crossing  the 


The  Diurnal  Motion 


L'l 


meridian.  Thus,  for  instance,  the  Nautical  Almanac  tells 
us  that  on  May  1st  1899  the  right  ascension  of  Vega  is 
18  hours  33  minutes  33-38  seconds.  This  means  that  in 
an  observatory  where  the  clock  is  going  properly  the 
time  thus  mentioned  is  the  time  at  which  Vega  would 
just  be  passing  behind  that  line  in  the  transit  instrument 
which  indicates  the  meridian. 

§  14.  Declination.  I  have  also  spoken  of  the  distance 
of  Vega  from  the  Pole  as  one  of  the  elements  to  be  deter- 
mined. Now  if  we  draw  a  line  from  the  Pole  through 
Vega  to  the  Equator  that  arc  from  the  Pole  to  the  Equator 
is  of  course  an  arc  of  90°,  and  the  part  of  it  between  Vega 
and  the  Equator  is  what  we  call  the  declination  of  Vega. 
Astronomers  have  been  led  to  adopt  the  declination  of  the 
star  rather  than  its  polar  distance  as  the  element  to  be 
recorded,  and  consequently  the  polar  distance  to  which 
I  have  already  referred  must  be  subtracted  from  90°  in 
every  case.  This  gives  us  the  decimation  which,  with 
the  right  ascension  as  already  explained,  completely 
defines  the  position  of  the  star. 

§  15.     The  Altitude  of  the  Pole  is  equal  to  the  Lati- 
tude.    It  is  easy  to  shew  that  the  altitude  of  the  Pole 
or  its  angular  distance  above  the  horizon  will  be  different 
at   different  places.     If,  for 
instance,   the  observer  were 
standing  on  the  North  Pole 
of  the  Earth,  then  it  is  plain 
that  the  North  Pole  of  the 
heavens   would   be    directly 
over  his  head.     His  latitude 
would  in  that  case  be  90°  and 
the  altitude  of  the  Pole  is 
precisely  the  same,  90°.     Or, 
on   the   other    hand,   if   the       Fig.  7_    Meaning  of  a  point 
observer  were  on  the  Equator          on  the  Celestial  Sphere. 


'2'1  Astronomy 

he  would  find  the  Pole  of  the  heavens  down  at  the  horizon, 
the  altitude  of  the  Pole  would  be  zero,  and  of  course  the 
observer  being  on  the  Equator  has  his  latitude  zero.  As 
we  have  seen  earlier  in  this  chapter,  the  distances  of  the 
stars  are  so  vast  compared  with  the  dimensions  of  the 
Earth  that  we  may  consider  the  whole  Earth  as  a  single 
point  at  the  centre  of  the  celestial  sphere. 

Or,  looking  at  the  question  from  another  point  of  VICAV, 
the  lines  drawn  from  any  two  points  of  the  Earth — say,  two 
opposite  points  of  the  Equator  E,  Q  (Fig.  7) — parallel  to 
the  axis  of  rotation  of  the  Earth,  will  remain  at  the  same 
distance  apart  however  far  they  may  be  produced.  They 
will  therefore  pierce  the  celestial  sphere  at  two  points 
separated  by  a  distance  equal  to  the  Earth's  diameter. 
But  at  the  distance  of  even  the  nearest  of  the  fixed  stars 
a  body  the  size  of  the  Earth  would  dwindle  to  the  most 
insensible  dimensions,  and  two  points  on  the  sphere  sepa- 
rated by  a  distance  even  a  thousand  times  greater  than  we 
have  supposed  could  not  possibly  be  distinguished  apart. 
We  therefore  arrive  at  the  important  conclusion  that  all 
parallel  lines  drawn  from  the  surface  of  the  Earth  appear 
to  meet  the  celestial  sphere  in  the  same  point. 

In  the  adjoining  figure  the  ellipse  EOPO'E'P  re- 
presents a  meridian  of  the  Earth,  P  and  P  are  the  nortli 
and  south  poles,  EE'  through  the  centre,  at  right  angles  to 
PP1,  is  the  equator,  and  0  is  the  position  of  the  observer. 
Then  if  the  line  ZON  represents  the  position  in  which  a 
plumb-line  at  0  will  hang,  OZ  is  the  direction  of  the 
vertical  and  OH  at  right  angles  to  it  is  a  section  of  the 
horizon  at  0.  Also  it  follows  from  the  last  paragraph 
that  the  line  Op,  drawn  through  0  parallel  to  PP',  is  the 
apparent  direction  of  the  Pole  as  seen  from  0.  Both  of 
these  directions  can  be  experimentally  determined  by  the 
observer  at  0.  The  direction  of  the  vertical  is  found  by  the 
actual  use  of  a  plumb-line  or  some  equivalent  apparatus  for 


Tlte 


M<>t!<n, 


determining  the  direction  of  gravity,  while  the  direction  of 
the  Pole  is  ascertained  by  observing  a  circumpolar  star 
above  and  below  the  Pole  as  explained  on  p.  19.  The  angle 
EXO  is  actually  the  geographical  latitude  of  0 — it  is,  in 
fact,  by  such  observations  that  the  latitude  is  determined 
—  and  the  angle  If  Op,  which  is  obviously  equal  to  it,  is 


Fig.  8.     Latitude  on  a  Spheroidal  Earth. 

the  altitude  of  the  Pole.  In  the  figure  a  second  point  O1 
has  been  taken  and  the  corresponding  lines  are  denoted 
by  accented  letters.  It  will  at  once  be  seen  that  the 
angle  E'X'O'  is,  as  in  the  former  case,  equal  to  IFO'p', 
which  is  of  course  the  altitude  of  the  Pole  at  0'. 


CHAPTER   II 

THE  SUN 

§  16.  Size  and  Importance  of  the  Sun.  Our  concep- 
tions of  the  scale  on  which  the  Universe  is  built  will 
perhaps  be  best  illustrated  at  the  commencement  by  con- 
sidering a  few  facts  with  regard  to  the  size  and  the  dis- 
tance of  the  Sun.  The  diameter  of  the  great  globe  is 
866,000  miles.  To  realise  fully  all  that  this  implies  we  must 
consider  that  it  is  more  than  a  hundred  times  as  great  as  the 
diameter  of  the  Earth.  Even  this  fact  will  however  hardly 
enable  us  to  form  an  adequate  conception  of  the  vast  dimen- 
sions of  the  Sun,  unless  we  further  take  into  account  that 
the  volume  of  a  sphere  is  proportional  to  the  cube  of  its 
diameter.  It  therefore  follows  that  if  the  Sun's  diameter 
is  more  than  a  hundred  times  that  of  the  Earth,  the  former 
body  must  be  more  than  a  million  times  greater  than  the 
Earth  in  volume.  This  fact  will  sufficiently  illustrate  the 
proper  perspective  in  which  our  Earth  is  to  be  placed  as  a 
body  in  the  Universe.  It  is  clearly  of  dimensions  which 
must  be  regarded  as  excessively  small  in  comparison  with 
those  of  the  great  luminary. 

The  distance  between  the  Earth  and  the  Sun  is  400 
times  greater  than  the  distance  of  the  Moon.  It  is  the  fact 
of  the  Sun's  being  so  situated  that  makes  it  appear  to  us 
24 


The  Sun  25 

no  larger  than  the  Moon,  which  is,  as  we  shall  see,  one  of 
the  smallest  bodies  in  the  System.  If  we  take  a  globe 
one  inch  in  diameter  to  represent  the  Earth,  then  we  must 
have  a  globe  9  feet  in  diameter  at  a  distance  of  323  yards 
to  represent  the  Sun  on  the  same  scale. 

§  17.  Mass  and  Density.  It  may  be  a  little  surprising 
to  note  that  though  the  Sun  is  more  than  a  million  times 
as  big  as  the  Earth,  yet  it  is  not  so  heavy  as  that  propor- 
tion would  imply.  If  the  Sun  were  made  of  materials 
which  were  of  the  same  nature  and  in  the  same  condition 
as  those  materials  of  which  our  Earth  is  composed,  then, 
seeing  that  the  Sun  is  more  than  a  million  times  larger  than 
the  Earth,  we  might  reasonably  expect  that  it  would  be 
heavier  in  a  corresponding  degree.  But  this  is  not  the  case. 
The  Sun  is  only  about  three  hundred  thousand  times  as 
heavy  as  our  Earth,  and  therefore  we  infer  from  these  fig- 
ures that,  bulk  for  bulk,  the  Sun  is  composed  of  materials 
which,  in  their  present  condition,  are  on  the  average  only 
about  one-third  of  the  weight  of  the  materials  of  the 
Earth's  globe. 

§  18.  The  Light  and  Heat  of  the  Sun.  The  extraordi- 
nary abundance  in  which  light  and  heat  are  emitted  from 
the  Sun  is  one  of  the  most  impressive  facts  in  Nature. 
The  distance  of  the  Sun  from  our  Earth  varies  slightly  at 
different  times  of  the  year,  but,  on  an  average,  it  has  been 
found  to  be  ninety-two  millions  nine  hundred  thousand 
miles.  When  we  reflect  how  quickly  the  warmth  and  the 
brightness  from  any  source  of  light  or  heat  decreases  as  the 
distance  of  that  source  increases,  we  do  indeed  find  roomf  or 
astonishment  at  the  quantity  and  the  intensity  of  the  light 
and  heat  from  the  Sun,  which  across  that  ninety -twoniillion 
nine  hundred  thousand  miles  is  able  to  transmit  to  us  such 
warmth  and  brilliance  as  we  enjoy  on  a  summer's  day. 

§  19.  Figure  of  the  Sun.  Sunspots.  The  circular 
form  which  the  Sun  presents  to  the  eye  is  due  to  the  circum- 


20  Astronomy 

stance  that  the  Sun  is  indeed  a  globe  and,  as  such,  has  a 
circular  outline  from  whatever  point  of  view  it  may  be 
observed.  The  first  step  in  the  investigation  of  the 
structure  of  the  Sun  raises  the  fundamental  question  as 
to  the  phvsical  nature  of  the  materials  of  which  it  is 
composed.  Are  they  solid,  liquid  or  gaseous  ?  It  might 
be  thought  that  the  Sun  is  a  solid  globe  like  the  Moon, 
but  raised  to  a  heat  so  intense  that,  instead  of  being  dark, 
and  opaque  like  the  Moon,  it  glows  with  vivid  incan- 
descence. But  the  telescope  shews  on  a  little  closer  exam  i- 
nation  that  this  view  must  be  abandoned.  When  we  study 
the  Moon  with  the  telescope  we  find  on  its  surface  defi- 
nitely marked  features  and  we  always  see  these  features 
in  the  same  position  whenever  the  observations  are  made. 
The  objects  characteristic  of  the  Moon  may  therefore  be 
described  as  permanent.  But  there  is  nothing  of  a  per- 
manent nature  on  the  Sun.  Frequently  the  brilliant  surface 
has  but  little  to  attract  attention  upon  it ;  occasionally, 
however,  it  will  be  found  marked  with  dark  spots. 

"The  very  source  and  fount  of  day 
Is  dashed  with  wandering  isles  of  night." 

The  appearance  of  these  "sunspots,"  for  so  they  are 
called,  is  well  exhibited  in  the  photograph  which  is  here 
reproduced,  which  was  taken  at  the  Eoyal  Observatory, 
(Greenwich,  on  Feb.  13, 1892.  Sunspots  vary  greatly  both 
as  to  size  and  form,  and  exhibit  various  degrees  of  perma- 
nence. Sometimes  they  endure  but  for  a  few  days  or  weeks ; 
sometimes  they  last  for  months.  It  is  thus  shewn  that  the 
Sun  cannot  be  a  solid  body.  Xo  solid  body  could  exhibit 
such  variable  features  as  are  the  solar  spots.  AVhen  the 
photograph  is  carefully  examined  it  is  seen  also  that  the 
texture  of  the  outer  covering  of  the  globe  is  by  no  means 
uniform.  The  surface  of  the  Sun  consists  of  briiliant  white 
granular  parts  floating  over  a  darker  interior,  and  the 


THE    SUN 
ROYAL  OBSERVATORY,  GREENWICH,  Feb.  13, 1892 

To  face  page  26 


Tit,-  Sun  27 

glimpses  of  that  darker  interior  which  we  occasionally 
obtain  reveal  to  ns  the  same  characteristics  as  the  dark 
centres  of  the  spots. 

§  20.  Rotation.  Careful  study  of  the  spots  has  dis- 
closed a  very  interesting  circumstance  connected  with  the 
Sun.  The  spots  as  we  have  said  occasionally  wax  and 
wane  and  sometimes  they  have  small  movements  of  their 
own  across  the  Sun's  surface.  But  it  was  found  by  Scheiner, 
so  long  ago  as  1627,  that  the  various  spots  all  partake  of  a 
common  movement  which  could  not  be  accounted  for  by 
the  supposition  of  any  proper  motion  in  the  spots  them- 
selves. It  is  invariably  found  that  they  move  across  the 
Sun  from  the  East  to  the  West.  Those  spots  which 
remain  visible  long  enough  to  enable  the  observation  to 
be  made  require  about  thirteen  or  fourteen  days  for  the 
journey  across  the  face  of  the  luminary.  Then  they 
require  as  much  more  for  the  journey  round  that  side  of 
the  Sun  which  is  turned  away  from  us.  And  then  they 
appear  again  at  the  eastern  margin  of  the  Sun.  This 
movement  is  so  universal  and  so  regular  that  it  is  obviously 
independent  of  the  movements  of  the  spots  themselves  on 
the  Sun.  We  find  a  complete  explanation  of  it  by  sup- 
posing that  the  Sun,  in  this  respect  like  our  Earth,  rotates 
011  an  axis  which  is  nearly  at  right  angles  to  the  ecliptic 
in  a  period  of  about  twenty-five  days.  We  thus  notice 
that  the  movements  of  the  Sun  are  much  slower  than  the 
terrestrial  movements,  inasmuch  as  the  Sun  requires  for 
each  revolution  a  period  about  25  times  as  long  as  tin- 
Earth.  It  should,  however,  be  pointed  out  that  the  period 
of  rotation  as  indicated  by  the  motion  of  the  spots  is  not 
the  same  for  all  portions  of  the  solar  surface.  Spots  at  the 
solar  equator  rotate  in  about  25  days.  Between  20°  and 
30°  of  solar  latitude  the  period  is  26  days,  while  spots 
which  break  out  at  40°  from  the  equator  require  27  days 
to  perform  ;i  revolution.  This  remarkable  fact  affords 


28  Astronomy 

another  proof  that  the  Sun's  globe  is  not  composed  of 
solid  materials. 

When  we  consider  the  enormous  preponderance  in  .the 
size  of  the  Sun  over  the  size  of  the  Earth,  there  is  no 
reason  to  be  surprised  at  the  fact  that  the  Sun's  move- 
ment of  rotation  should  be  so  much  slower  than  that  of 
our  globe.  Even  as  it  is,  the  equatorial  parts  of  the  Sun 
are  actually  whirled  along  three  times  as  fast  as  the  equa- 
torial parts  of  our  Earth. 

§  21.  Periodic  Changes  in  Sunspots.  One  of  the  most 
enigmatical  matters  connected  with  the  sunspots  is  the 
fact  that  the  number  in  which  they  are  present  appears 
to  undergo  a  periodical  change.  A  German  astronomer, 
Schwabe,  commenced  in  1826  a  regular  study  of  the 
sunspots,  and  after  many  years  of  painstaking  labour 
devoted  to  this  subject,  he  was  able  to  bring  under  the 
domain  of  law  the,  at  first  sight,  irregular  variations  in 
the  outbursts  of  sunspots,  and  to  determine  the  length  of 
the  period  in  which  those  variations  occurred.  From  his 
investigations,  as  well  as  from  the  subsequent  labours 
Avhich  have  been  devoted  to  this  siibject,  the  length  of  the 
cycle  has  been  ascertained  to  be  about  eleven  years  and 
five  weeks.  That  is  to  say,  if  we  measure  from  a  time 
when  the  sunspots  are  at  their  greatest  both  as  to  number 
and  to  individual  size,  we  find  that  in  eleven  years  and 
five  weeks  there  is  a  recurrence  generally  speaking  of  the 
same  conditions.  In  the  course  of  that  period,  the  num- 
ber of  sunspots  undergoes  remarkable  changes.  Six  or 
seven  years  after  the  time  of  maximum  the  sunspots  are 
reduced  to  a  minimum.  In  some  cases  they  vanish  alto- 
gether, and  then  a  gradual  increase  in  the  number  takes 
place  until  the  ensuing  maximum  is  attained. 

§  22.  Connexion  between  Sunspots  and  Terrestrial 
Magnetism.  A  connexion  is  now  believed  to  exist  be- 
tween the  number  of  sunspots  on  the  surface  of  the 


The  Sun  29 

Sun,  and  the  variations  of  the  magnetic  needle  on  the 
Earth.  This  remarkable  phenomenon  presents  itself  in 
various  ways  and  I  may  mention  one  in  particular.  By 
the  magnetic  declination  we  mean  the  angle  between  the 
direction  of  the  Xorth  Pole  and  the  direction  in  which 
the  magnetic  needle  points  if  hnng  in  such  a  way  that  it 
is  free  to  move  around  a  vertical  axis.  At  Greenwich,  for 
instance,  the  needle  points  about  17°  West  of  North,  and 
the  declination  is  accordingly  said  to  be  about  17°.  It  has 
been  found  that  the  declination  is  not  constant.  It  varies 
in  different  ways,  and  in  particular  it  has  a  slow  daily 
oscillation  to  and  fro.  Further,  the  extent  to  which  the 
needle  oscillates  in  the  course  of  a  day  on  either  side  of 
its  mean  position  is  found  to  vary.  But  the  remarkable 
circiim  stance  is  that  the  extent  of  the  diurnal  variation 
reaches  a  maximum  at  the  time  when  the  sunspots  are 
greatest. 

§  23.  The  Spectroscope.  Many  of  the  most  remarkable 
advances  in  modern  astronomy  are  connected  with  the 
study  of  the  Sun.  The  information  which  the  telescope 
lias  given  us  with  regard  to  our  luminary  has  been  sup- 
plemented in  a  most  astonishing  manner  by  the  revelations 
of  the  spectroscope.  We  must  therefore  give  here  some- 
description  of  this  instrument  and  its  application  to  the 
study  of  celestial  bodies.  It  will  be  unnecessary  for  us 
to  enter  into  special  details  with  regard  to  the  construc- 
tion of  the  instrument,  or  the  mode  of  using  it  in  chemi- 
r;il  work,  since  the  spectroscopic  method  of  analysis  is 
discussed  in  every  modern  work  on  Chemistry. 

§  24.  Composition  of  Sunlight.  A  beam  of  light  from 
the  Sun,  though  it  looks  so  simple,  is  in  reality  of  a  most 
complex  nature.  A  sunbeam  consists  not  of  homogeneous 
white  light,  but  of  an  innumerable  multitude  of  rays  of 
light  of  different  hues,  the  combination  of  Avhich  gives  us 
the  colour  we  call  white.  These;  rnys  are  mingled  so  closely 


30  Asti'Oitunii/ 

that  the  complex  nature  of  the  sunbeam  would  never  be 
suspected  until  special  means  were  employed  to  separate 
it  into  its  different  elements. 

$  25.  The  Prism.  The  agent  that  we  use  for  the  decom- 
position of  the  beam  of  light  is  a  prism  of  glass.  It  is  true 
that  for  the  higher  purposes  of  modern  astronomy  the 
grating,  which  consists  of  a  plate  of  glass  ruled  with  an 
enormous  number  of  fine  lines  placed  very  close  together 

sometimes  as  many  as  20,000  to  the  inch  —  and  which 

effects  the  analysis  of  the  light  in  a  different  way,  is  per- 
haps the  more  efficient  instrument,  yet  for  our  present 
purposes  of  description  it  will  be  sufficient  to  refer  to  the 
prism.  This  course  will  appear  all  the  more  justifiable 
when  it  is  borne  in  mind  that  the  fundamental  discoveries, 
by  which  spectrum  analysis  when  applied  to  the  heavens 
enables  us  to  unravel  some  of  the  deeper  secrets  of  nature, 
have  been  mainly  due  to  the  prism,  the  grating  having 
come  into  operation  only  after  the  cardinal  discoveries  had 
already  been  made.  We  may  think  of  a  prism  as  a  piece 
of  pure  glass,  cut  into  a  wedge-shaped  form  with  perfectly 
flat  sides.  A  ray  of  light  from  the  Sun  or  indeed  from  any 
luminous  source  whatever,  when  it  falls  on  one  of  the  sides 
of  the  prism  and  passes  through,  undergoes  a  remarkable 
transformation  on  emerging  at  the  opposite  side.  In  the 
first  place  that  ray  of  light  is  bent,  or  refracted,  from  its 
original  track.  The  ray  is  bent  always  towards  the  thick 
part  of  the  prism.  If  however  the  action  of  the  prism 
consisted  in  merely  bending  all  rays  alike  it  coiild  never 
have  created  the  method  of  research  which  we  know  a.s 
spectrum  analysis. 

§  26.  Refraction  and  Dispersion.  If  a  beam  of  sun- 
light is  admitted  through  a  small  circular  aperture  and 
allowed  to  fall  upon  a  screen  it  will  appear  as  a  circular  spot 
of  light  upon  the  screen.  If  a  prism  is  interposed  in  the 
path  (Fig.  0)  not  only  is  the  spot  of  light  deflected  but  it  is 


The  Sun 


31 


drawn  out  from  the  circular  form  into  an  elongated  patch, 
coloured  red  at  one  end  and  violet  at  the  other,  with  the 
intermediate  shades  of  yellow,  green  and  blue  between. 
This  separation  of  the  colours  is  known  as  dispersion. 


Fig.  0.     Refraction  and  Dispersion. 

It  may  easily  be  shewn  that  if  we  were  dealing  with 
homogeneous  light  we  should  still  find  a  simple  circular 
spot  of  light  upon  the  screen.  In  other  words,  if  we  had 
red  light  we  should  find  a  circular  red  spot,  if  green  light 
a  circular  green  spot,  and  so  on.  Hence  the  conclusion  is 
obvious  that  the  coloured  patch  which  is  seen  when  sun- 
light is  passed  through  a  prism  is  due  to  the  overlapping 
of  these  variously  coloured  circular  spots. 

§  27.  Necessity  for  Narrow  Slit.  In  order  to  prevent 
this  overlapping  as  much  as  possible  and  thus  to  keep  the 
separate  rays  apart,  let  us  substitute  for  the  circular  hole 
a  very  narrow  slit  with  its  length  placed  parallel  to  the 
refracting  edge  of  the  prism,  and  we  shall  then  have  all 
the  essential  parts  of  a  spectroscope.  It  is  true  that  for 
delicate  researches  we  have  to  pass  the  beam  through  a 
lens,  called  a  collimator,  before  it  reaches  the  prism  and  to 
receive  it  in  a  small  viewing  telescope  instead  of  allowing 
it  to  fall  on  a  screen  when  it  emerges  on  the  other  side,  but 
what  we  see  is,  as  before,  a  band  of  colour  consisting  of 


32  Astronomy 

variously  coloured  images  of  the  slit  placed  very  close 
together,  merging  into  each  other  and  thus  forming  the 
beautiful  coloured  ribbon  of  light  which  we  call  the  solar 
spectrum. 

§  28.  The  Ether.  We  know  that  light  consists  of 
waves  or  undulations  in  the  ether,  that  subtle  material 
which  fills  all  space  and  permeates  all  bodies,  as  easily, 
to  use  the  famous  illustration,  as  the  "wind  blows  through 
a  grove  of  trees." 

We  know  too  that  the  colour  of  a  ray  depends  simply 
upon  the  wave-length  of  the  light  composing  it.  Thus 
the  prism  in  sorting  out,  as  we  have  seen,  all  the  differ- 
ent colours  that  go  to  compose  a  beam  of  white  light 
arranges  them  according  to  their  several  wave-lengths. 

It  will  thus  be  obvious  that  the  prism  provides  us  with 
a  means  of  examining  the  structure  of  any  rays  of  light 
which  are  emitted  from  the  Sun.  Several  precautions  of 
vast  importance  to  the  practical  astronomer  have  to  be 
attended  to,  but  they  are  of  a  nature  so  technical  that 
there  is  no  need  to  set  them  down  in  these  pages.  But 
when  these  precautions  have  been  observed,  we  are  pro- 
vided with  the  means  of  examining  separately  the  differ- 
ent rays  of  light  which  have  been  combined  together  into 
a  single  sunbeam. 

§  29.  Use  of  Photography,  hi  the  study  of  this  sub- 
ject, as  in  the  study  of  so  many  other  parts  of  modern 
astronomy,  the  help  of  photography  has  been  invoked 
Avith  great  success.  By  the  aid  of  photography  we  are 
enabled  to  obtain  in  an  absolutely  reliable  manner  the 
positions  of  many  of  the  rays  into  which  the  beam  has 
been  decomposed  by  the  action  of  the  prism.  But  great 
as  may  be  the  use  of  photography  in  procuring  a  record 
of  absolutely  unchallenged  accuracy,  the  service  it  renders 
to  the  astronomer  involves  much  more  than  would  be 
implied  by  this  statement,  as  will  now  be  explained. 


§  30.  Invisible  Rays.  \Ye  have  said  that  a  beam  of 
sunlight  consists  of  a  very  large  number  of  rays  of  distinct 
character  blended  together.  Those  rays  include  among 
them  rays  possessing  all  the  hues  of  the  rainbow.  But 
besides  those  there  are  other  rays.  We  have  all  heard  of 
people  who  are  affected  by  what  is  known  as  "  colour 
blindness."  Those  who  are  colour  blind  have  a  defect 
connected  with  the  nerves  by  which  certain  particular  rays 
carry  the  impression  from  the  retina  to  the  brain.  Some 
people  are  colour  blind  in  some  way,  and  some  in  another. 
The  introduction  of  photography  has  however  revealed  the 
astonishing  fact  that  humanity  generally  is  colour  blind. 
That  is  to  say,  that  there  are  many  rays  and  rays  of  much 
intensity  in  the  solar  spectrum,  which  are  of  such  a 
character  that  no  human  eye  can  see  them.  Thus  some 
people  are  blind  to  the  blue  rays,  others  to  the  red  rays, 
while  every  one  of  us  is  blind  to  a  whole  series  of  rays  lying 
beyond  the  violet  in  the  spectrum  whose  existence  the 
photographic  plate  announces.  It  is  indeed  a  noteworthy 
fact  that  some  of  the  rays  to  us  invisible  are  not  only 
visible  to  the  photographic  eye,  but  are  much  brighter 
photographically  than  the  rays  which  are  capable  of  affect- 
ing our  sense  of  vision.  This  circumstance  has  given 
extraordinary  emphasis  to  the  value  of  photography  for 
astronomical  purposes.  Not  alone  will  the  photograph 
receive  through  the  spectroscope  and  record  for  our  in- 
struction many  if  not  all  of  the  rays  which  we  can  see,  but 
it  will  take  rays  which  we  cannot  see,  and  which  we  have 
no  other  means  of  perceiving,  and  it  will  often  represent 
these  rays  with  a  distinctness  not  at  all  inferior,  and 
indeed  in  some  cases  very  much  superior,  to  the  distinct- 
ness with  which  it  sets  forth  the  rays  that  appeal  directly 
to  our  sense  of  vision. 

Let  us  suppose  that  we  desire  to  obtain  a  photograph 
of  the  spectrum  of  the  Sun.  The  light  is  admitted  to  the 


34  Astronomy 

photographic  spectroscope  as  into  the  visual  instrument  to 
which  we  have  already  referred  through  a  very  narrow  slit, 
which  is  placed  parallel  to  the  refracting  edge  of  the  prism. 
The  light  enters  therefore  in  the  form  of  a  very  thin  ribbon. 
This  thin  slice  of  light,  if  it  merely  fell  upon  the  plate, 
without  the  interposition  of  the  prism,  would  simply  give  a 
line  of  light.  The  action  of  the  prism,  however,  separates 
the  variously  coloured  lines  of  light  whose  juxtaposition 
makes  up  the  spectrum  band. 

§  31.  Fraunhofer  Lines.  The  first  remarkable  fact  to 
be  noticed  is  that  the  glorious  band  of  colour  from  the  ex- 
treme red  to  the  extreme  violet  is  crossed  by  numerous  dark 
lines.  These  were  discovered  long  before  their  nature  was 
understood.  They  were  noticed  by  Wollaston  about  the 
year  1802,  but  they  are  generally  known  by  the  term 
"  Fraunhofer  "  lines,  as  it  was  the  illustrious  physicist 
of  that  name  who  first  studied  them  with  care.  He  recog- 
nised that  these  lines  were  constantly  present  in  sunlight, 
he  saw  that  like  the  stars  in  the  heavens  they  had  their  dis- 
tinct positions  Avhich  they  permanently  preserved,  and  he 
saw  that  they  also  differed  greatly  in  their  intensity :  that 
they  were  in  fact  characteristic  features  in  the  solar  spec- 
trum. Perceiving  these  facts  he  attached  symbols  to  repre- 
sent these  different  lines.  The  strong  line  in  the  extreme 
red  he  called  A,  then  came  B  and  (7,  D,  a  very  famous  line, 
Avas  in  the  yellow,  and  then  followed  E,  F  and  G,  and 
lastly  //  in  the  violet.  Other  lines  have  been  since  added 
beyond  //,  but  these  are  in  that  part  of  the  spectrum 
Avhich  is  only  visible  to  the  photographic  plate. 

The  lines  so  designated  are  only,  however,  the  more 
conspicuous  ones  of  a  very  large  host.  I  have  already 
likened  the  lines  of  the  solar  spectrum  to  the  stars  in  the 
sky,  and  we  may  perhaps  carry  the  analogy  a  step  further. 
With  every  increase  in  our  optical  power  the  stars  in 
the  heavens  appear  in  ever  increasing  numbers.  In  like 


35 

manner  the  number  of  the  lines  in  the  solar  spectrum 
increases  enormously  with  every  improvement  in  the 
delicacy  and  power  of  the  apparatus  with  which  the  re- 
fraction is  produced,  and  with  every  advance  in  the  delicacy 
of  the  photographic  plate  on  which  the  pictures  are  re- 
ceived. At  first  the  lines  were  recognised  in  dozens  or 
in  scores,  now  they  are  known  to  the  number  of  many 
thousands.  It  has  frequently  happened  that  lines  which 
appeared  single  in  the  first  instance  have  by  closer  examina- 
tion been  shown  to  be  composed  of  two  or  more  distinct  lines 
so  near  as  to  appear  coincident.  And  this  process  of  the 
discovery  of  new  lines  is  still  going  on.  By  passing  the 
light  through  a  number  of  prisms  in  succession  the  dis- 
persive effect  of  a  single  prism  is  proportionately  multiplied 
and  the  length  of  the  spectrum  and  the  distance  of  the 
lines  from  each  other  can  thus  be  increased.  Since,  how- 
ever, the  light  is  drawn  out  thinner  and  thinner  with  every 
increase  of  dispersion  there  is  a  limit  to  the  dispersion 
which  can  be  usefully  employed.  The  extremely  sensitive 
plates  now  made  by  enabling  a  higher  dispersion  to  be  used 
have  greatly  increased  the  number  of  the  lines  in  the 
ultra-violet,  or  invisible,  regions,  and  many  of  the  most 
important  applications  of  spectroscopic  astronomy  depend 
for  their  success  upon  the  interpretation  of  the  lines  set 
down  on  the  photographic  plate  in  those  parts  of  the 
spectrum  which  are  entirely  beyond  the  reach  of  the 
human  eye. 

§  32.  Meaning  of  the  Dark  Lines  in  the  Solar  Spec- 
trum. The  explanation  of  the  meaning  of  these  dark 
lines  in  the  solar  spectrum  constitutes  one  of  the  most 
important  and  far-reaching  discoveries  which  have  been 
made  in  the  nineteenth  century.  The  phenomenon  Avhich 
has  to  be  accounted  for  is  (if  we  concentrate  our  attention 
solely  on  a  single  line),  that  in  the  composite  beam  of  light 
reaching  us  from  the  Sun,  the  light  of  just  that  special 


30 

wave-length,  which  would  go  to  form  that  part  of  the 
spectrum  where  the  line  occurs,  is  wanting.  The  light  of 
wave-length  a  little  greater  is  there,  the  light  of  wave- 
length a  little  less  is  also  there.  But  that  particular 
wave-length  is  not  represented.  How  delicate  the  phe- 
nomenon is  will  be  realised  if  we  consider  that  even  though 
the  spectrum  were  a  foot  long  yet  the  line  in  question  is  so 
tine  that  it  would  hardly  be  possible  with  the  finest  pen  to 
rule  a  line  across  that  spectrum  which  would  not  be  too 
coarse  to  do  justice  to  it.  The  point  then  to  explain  is  the 
absence  in  this  marked  and  most  emphatic  manner  of  a 
particular  ray  of  light. 

We  know  that  the  brilliant  parts  of  the  Sun,  those 
parts  which  send  to  us  the  light  and  the  heat  so  necessary 
for  our  welfare,  really  transmit  rays  of  every  description, 
and  consequently  the  light  which  they  emit  if  that  light 
could  be  received  by  us  before  it  undergoes  any  subsequent 
treatment  would  present  what  we  call  a  continuous  spec- 
trum unmarked  by  any  of  these  dark  lines.  The  dark 
lines  take  their  origin  in  something  which  happens  to  the 
ray  of  light  after  it  has  left  the  brilliant  part  of  the  Sun 
and  before  it  reaches  our  instruments.  The  fact  is  that 
the  Sun  is  surrounded  with  an  atmosphere,  an  atmosphere 
to  us  invisible,  but  extending  to  a  great  height  above  that 
brilliant  region  in  which  the  light  and  heat  have  their 
origin.  The  rays  from  the  Sun,  or  rather  from  the  brilliant 
parts  of  it,  have  to  traverse  this  atmosphere.  This  atmos- 
phere is,  as  I  have  said,  invisible.  It  is  so  transparent 
that  it  permits  the  passage  of  the  light  without  much 
appreciable  alteration  in  its  intensity.  But  it  does  not 
allow  the  light  to  pass  through  without  producing  some 
effect  upon  it.  And  the  effect  is  of  a  very  remarkable 
character.  It  vigorously  opposes  the  motion  through  it  of 
light  of  certain  particular  wave-lengths  while  allowing  a 
comparatively  unobstructed  passage  to  the  light  of  every 


The  N««  87 

other  description.  This  is  indeed  a  very  remarkable 
property  of  a  transparent  atmosphere,  but  on  its  recogni- 
tion depend  many  of  the  most  interesting  of  modern  dis- 
coveries. The  rays  thus  obstructed  are  not  all  connected 
together,  they  are  in  different  parts  of  the  spectrum.  The 
consequence  is  that  the  spectrum  of  the  Sun  is  marked 
with  numbers  of  dark  lines,  numbers  indeed  amounting  to 
many  thousands,  and  the  more  conspicuous  of  these  are 
what  are  known  as  the  Fraunhofer  lines. 

And  now  for  the  interpretation  of  these  dark  lines. 
Each  line,  or  rather  each  group  of  lines,  is  due  to  the 
presence  of  some  particular  element  in  the  solar  atmos- 
phere. I  may  take,  as  a  case  for  illustration,  that  most 
common  but  important  element,  iron.  The  abundance  of 
iron  on  our  Earth  seems  to  be  paralleled  by  its  abundance 
on  some  of  the  celestial  bodies,  the  Sun  itself  included. 
Owing  however  to  the  great  heat  of  the  Sun,  the  iron  in 
that  mighty  furnace  has  not  only  been  fused  to  a  liquid 
form,  but  it  has  actually  been  boiled  from  the  liquid  into 
the  gaseous  form,  so  that  the  iron  in  the  Sun  so  far  at 
least  as  this  element  concerns  us  at  present,  is  to  be 
regarded  as  a  gas,  diffused  throughout  the  solar  atmos- 
phere. This  iron  vapour,  could  we  view  it  under  ordinary 
circumstances,  would  be  regarded  as  transparent.  It  would 
let  the  light  pass  through  without  appreciable  diminution. 
But  when  a  closer  examination  is  instituted,  and  when  we 
study  the  action  of  the  iron  vapour  on  the  individual  rays 
of  different  wave-lengths  with  the  aid  of  the  spectroscope, 
we  then  find  that  the  vapour  exercises  what  we  may  almost 
describe  as  quite  an  arbitrary  power  of  absorption.  The 
majority  of  rays  are  allowed  to  pass  unmolested;  they 
pass  as  freely  as  ordinary  sunlight  passes  through  a  pane 
of  glass.  But  there  are  certain  particular  wave-lengths 
which  are  in  some  way  or  other  specially  related  to  the 
movements  of  the  molecules  of  iron  vapour,  and  to  waves 


429004 


38  Astronomy 

which  possess  these  particular  wave-lengths  no  passage 
is  permitted.  To  hues  of  these  particular  kinds  the  iron 
atmosphere  is  specially  opaque;  they  are  not  allowed  to 
pass,  and  there  are  thousands  of  such  rays.  The  conse- 
quence is  that  in  the  solar  spectrum  among  the  innumer- 
able lines,  there  are  a  number,  to  be  counted  in  thousands, 
which  are  due  to  the  presence  of  iron  in  the  solar  atmos- 
phere. 

§  33.  Coincidence  of  the  Bright  Lines  of  Emission,  with 
the  Dark  Lines  of  Absorption  Spectra.  A  very  remarkable 
circumstance  has  now  to  be  mentioned.  Suppose  that 
we  introduce  two  pieces  of  pure  iron  as  the  poles  of  an 
electric  light.  In  the  intense  heat  of  the  electric  arc  the 
iron  is  not  only  fused,  but  it  is  driven  into  vapour,  and 
that  vapour  is  brilliantly  incandescent.  If  the  light 
emitted  from  this  electric  arc  is  viewed  through  a  spec- 
troscope, a  spectrum  is  seen  which  is  fundamentally  dif- 
ferent from  the  spectrum  of  a  beam  of  sunlight.  In  the 
latter,  as  we  have  said,  there  are  the  continuous  colours 
of  the  rainbow  ruled  over  by  innumerable  fine  dark  lines. 
But  the  spectrum  of  the  iron  vapour  from  the  poles  of  the 
incandescent  iron  exhibits  a  spectrum  of  a  totally  different 
character.  In  this  case  we  see  a  number  of  distinct  bright 
lines,  while  the  continuous,  gorgeously  coloured  band  of 
rainbow  hues  is  quite  wanting.  By  a  suitable  contrivance 
the  spectrum  of  the  Sun  can  be  conducted  into  the  same 
instrument  as  that  in  which  the  spectrum  from  the  incan- 
descent iron  poles  is  viewed.  In  the  one  case  we  have 
the  brilliant  band  with  the  thin  dark  lines,  in  the  other 
case  we  have  no  brilliant  band,  but  a  large  number  of  thin 
bright  lines.  The  extraordinary  fact  is,  that  when  these 
two  spectra  are  placed  in  juxtaposition,  the  position  of 
each  bright  line  in  one  spectrum  is  found  to  tally  exactly 
with  the  position  of  certain  dark  lines  in  the  other.  This 
coincidence  is  of  a  most  striking  nature.  It  would  be 


Tli?  s.i,,  39 

remarkable  enough  if  it  occurred  in  the  case  of  one  or  two 
lines  only,  but  when  we  find  each  one  of  the  multitude  of 
lines  in  the  artificial  iron  spectrum  agreeing  to  the  last 
degree  of  precision  with  the  corresponding  line  in  the  solar 
spectrum,  it  becomes  altogether  inconceivable  that  such 
coincidences  could  be  the  result  of  accident.  The  explana- 
tion of  this  most  astonishing  phenomenon  seems  to  have 
been  first  suggested  by  Sir  George  Stokes,  but  the  general 
and  far  reaching  law  of  which  it  is  an  instance  was  dis- 
covered and  enunciated  by  Kirchhoff  about  the  year  1860. 
It  is  now  known  that  when  iron  vapour  is  heated  to  such 
a  degree  of  brilliance  that  it  pours  forth  luminous  vibra- 
tions, the  character  of  the  light  that  emanates  from  it  is 
precisely  the  same  as  that  of  the  light  which  iron  vapour 
is  capable  of  arresting.  We  may  indeed  illustrate  this 
remarkable  property  in  this  way.  Iron  vapour  generally 
speaking  is  almost  transparent.  It  levies  scarcely  any  toll 
on  the  light  which  passes  through  it,  except  on  rays  of 
certain  particular  wave-lengths.  When,  on  the  other 
hand,  this  iron  vapour  is  incandescent,  as  it  is  in  the 
lower  atmosphere  of  the  Sun,  the  light  that  it  pours  forth 
is  composed  exclusively  of  rays  of  those  particular  kinds 
which  are  absorbed  in  the  former  case. 

Illustrations  connected  with  music  may  be  given  of  this 
principle.  The  strings  in  a  piano  when  musical  notes  are 
sounded  in  their  vicinity  will  severally  respond  to  those 
vibrations  which  are  tuned  to  harmonise  with  them.  And 
the  particular  note  that  each  wire  will  absorb  is  precisely 
that  same  note  which  it  emits  when  struck. 

§  34.  Relation  between  Absorption  and  Emission.  I  have 
selected  the  case  of  iron  merely  as  an  illustration.  What 
has  been  said  with  regard  to  this  metal  may,  with  cer- 
tain modifications,  be  stated  of  any  other  element.  Each 
element  present  in  the  Sun's  vapour  absorbs  light  of 
precisely  those  refrangibilities  which  that  element  would 


40  Afitrunotny 

emit  when  heated.  Here  then  we  have  the  key  to  the 
interpretation  of  these  wonderful  dark  lines  in  the  solar 
spectrum.  The  astronomer  who  endeavours  to  account 
for  these  lines  produces  artificially,  by  the  help  of  the 
electric  arc,  the  spectra  of  the  different  metals  and  other 
elements.  The  lines  in  these  spectra  are  compared  with 
the  lines  in  the  solar  spectrum.  When  a  case  of  coinci- 
dence is  sufficiently  made  out,  a  coincidence  based  not 
merely  on  an  agreement  between  a  few  of  the  lines,  but 
on  the  substantial  agreement  of  the  whole  system,  then 
we  have  a  demonstration  of  the  existence  of  the  corre- 
sponding element  as  one  of  the  solar  constituents.  The 
cogency  of  this  reasoning  is  very  impressive  to  any  one 
who  has  had  an  opportunity  of  witnessing  the  extreme 
delicacy  with  which  the  lines  artificially  produced  will 
coincide  with  the  corresponding  dark  lines  in  the  solar 
spectrum. 

§  35.  The  "  D  "  Line  of  Sodium.  The  best  known  in- 
stance of  this  coincidence  relates  to  the  famous  line  "  D," 
as  Fraunhofer  designated  it,  in  the  solar  spectrum.  This 
line  is  in  the  orange  part  of  the  band  of  light,  and  is  com- 
posed of  a  pair  of  lines  very  close  together.  The  juxta- 
position of  these  very  close  lines  is  in  itself  a  remarkable 
feature.  Indeed  an  instrument  of  some  power  is  required 
to  shew  that  the  two  lines  are  separate.  When  the  com- 
parison is  made  between  the  solar  spectrum  and  these  two 
lines  which  are  due  to  the  element  sodium,  and  can  be  pro- 
duced by  simply  placing  a  little  salt  in  the  flame  of  a 
spirit-lamp  held  in  front  of  the  slit,  or  by  some  equivalent 
method,  the  striking  coincidence  between  the  two  bright 
lines  of  the  element  and  the  two  dark  lines  in  the  solar 
spectrum  is  at  once  observed.  It  is  easy  to  demonstrate 
that  the  chances  are  millions  to  one  in  favour  of  such  a 
coincidence  being  due  to  a  physical  cause,  and  not  being 
merely  accidental.  The  lines  seen  in  the  spectrum  of  sun- 


TOTAL    SOLAR   ECLIPSE,   1893 

{Shewing  Prominences} 
SCHAEBERLE 


To  face  page  40 


Tin'  Sun  -41 

light  thus  afford  the  clearest  evideiice  of  the  existence  of 
the  corresponding  elements  in  the  Sun's  atmosphere. 

§  36.  Chemical  Elements  in  the  Sun.  This  method  is 
employed  not  only  in  the  study  of  the  Sun  but  it  has  been 
also  utilised  for  investigating  the  physical  constitution  of 
the  stars,  the  comets,  and  the  nebulae.  In  fact,  it  is  not 
too  much  to  say  that  the  whole  of  modern  astronomy  has 
been  largely  influenced  by  this  new  method  of  investiga- 
tion. Among  the  elements  known  on  the  Earth  which 
have  been  proved  by  this  method  to  be  present  in  the  Sun, 
we  may  mention  calcium,  iron,  hydrogen,  sodium,  carbon, 
nickel,  magnesium,  cobalt,  aluminium,  chromium,  stron- 
tium, manganese,  copper,  zinc,  cadmium,  silver,  tin,  lead, 
potassium. 

§  37.  Appearances  during  a  Total  Eclipse  of  the  Sun. 
We  are  also  largely  indebted  to  the  spectroscope  for 
information  about  some  features  of  the  Sun  which  are 
too  faint  to  be  seen  in  the  conditions  of  ordinary  daylight, 
and  which,  without  its  aid,  are  visible  only  when  the 
excessively  brilliant  part  of  the  Sun  is  obscured  during  a 
total  eclipse.  A  total  eclipse  occurs  on  those  rare  occa- 
sions when  the  body  of  the  Moon  is  placed  directly  be- 
tween the  observer  and  the  Sun.  It  happens  by  a  curious 
coincidence  that  the  apparent  diameter  of  the  Moon  so 
closely  approximates  to  the  apparent  diameter  of  the  Sun 
that  when  the  Moon  is  centrally  placed  it  only  just  stops 
out  the  brilliant  body  of  the  Sun,  but  happily  leaves  for 
ouv  inspection  the  marginal  fringe  round  the  great  lumi- 
nary, in  which  those  delicate  objects  are  contained  which 
arc  ordinarily  withheld  from  our  inspection. 

$  38.  Prominences.  The  solar  features  with  which  we 
are  at  present  concerned  arc  the  prominences  and.  the  corona.. 
The  prominences  are  usually  flame-like  projections  of  a 
ruddy  colour,  which  often  extend  for  many  thousands  of 
miles  above  the  surface  of  the  Sun.  They  can  never  be 


witnessed  by  the  unaided  eye,  nor  even  with  the  help  of 
a  telescope,  except  when  an  eclipse  takes  place.  It  was 
however  discovered  simultaneously  and  independently  by 
Professor  Jansseii  and  Sir  Norman  Lockyer  that  with 
the  help  of  the  spectroscope  the  prominences  could  be 
observed  at  any  time  even  without  an  eclipse.  This  inter- 
esting advance  in  astronomical  art  is  due  to  the  circum- 
stance that  these  prominences  being  mainly  composed  of 
incandescent  gas,  emit  a  light  which,  unlike  the  general 
light  of  the  Sun,  does  not  contain  all  degrees  of  refrangi- 
bility,  but  merely  consists  of  two  or  three  refrangibilities. 
The  consequence  is  that  by  the  help  of  the  spectroscope  the 
sunlight  surrounding  these  objects  can  be  diffused  along 
the  whole  length  of  the  spectrum,  and  thus  be  consider- 
ably weakened  so  that  the  light  of  the  prominence  con- 
centrated in  two  or  three  lines  is  no  longer  overwhelmed 
as  it  is  under  ordinary  circumstances  by  the  brilliance  of 
ordinary  sunlight,  and  consequently  makes  itself  visible. 

§  39.  Corona.  The  other  feature  in  the  solar  sur- 
roundings which  is  witnessed  during  an  eclipse  is  the 
Corona.  This  delicate  pearl-like  light  has  never  yet  been 
seen  by  any  artifice  except  on  the  occasion  of  an  eclipse. 
It  is  found  that  the  Corona  presents  some  bright  lines  due 
to  the  presence  of  incandescent  gases,  one  of  which  is 
attributed  to  some  element,  "  coronium,"  of  which  indeed 
but  little  is  known.  It  is  also  evident  from  the  photo- 
graph of  its  spectrum  that  the  Corona  contains  a  good 
deal  of  material  in  suspension  of  the  nature  of  dust  or 
fog,  for  otherwise  we  could  not  account  for  the  fact  that  it 
has  a  faint  continuous  spectrum,  as  of  reflected  sunlight. 

§  40.  Depth  ef  the  Photosphere.  The  light  of  the  great 
orb  of  day  emanates  almost  exclusively  from  one  single 
layer  of  surpassing  brightness  called  the  photosphere.  The 
great  bulk  of  the  Sun  which  lies  within  that  brilliant 
mantle  is  comparatively  obscure,  and  seems  to  play  but  an 


TOTAL   SOLAR    ECLIPSE.    1898 

(Shewing  Corona) 
MICHIE  SMITH 


To  face  page  42 


43 

unimportant  part,  so  far  as  the  immediate  dispensing  of 
light  and  heat  is  concerned.  In  order  to  give  an  idea  of 
the  thickness  of  this  photosphere  in  comparison  with  the 
Snn's  diameter  we  might  liken  his  brilliant  exterior  to 
the  rind  of  an  orange,  while  the  gloomy  interior  regions 
would  correspond  to  the  edible  portion  of  the  fruit. 
( lenerally  speaking,  the  rind  of  the  orange  is  rather  too 
coarse  for  the  purpose  of  this  illustration ;  it  might  be 
nearer  the  truth  to  affirm  that  the  luminous  part  of  the 
Sun  may  be  compared  in  thickness  with  the  delicate  filmy 
skin  of  the  peach.  There  can  be  no  doubt  that  if  this 
glorious  mantle  were  unhappily  stripped  from  the  Sun 
the  great  luminary  would  forthwith  disappear  and  cease 
to  possess  the  power  of  shedding  abroad  light  and  heat. 
The  spots  which  so  frequently  seem  to  fleck  the  dazzling 
surface  are  probably  mere  rents  in  the  brilliant  mantle 
through  which  we  are  permitted  to  obtain  glimpses  of 
the  non-luminous  interior.  It  should,  however,  be  said 
that  a  full  and  satisfactory  explanation  of  all  the  phe- 
nomena attending  these  curious  and  interesting  objects 
has  not  yet  been  attained. 

§  41.  Materials  composing  the  Photosphere.  As  the 
ability  of  the  Sun  to  warm  and  light  this  Earth  arises 
from  the  peculiar  properties  of  the  thin  glowing  shell 
which  surrounds  it,  a  question  of  the  greatest  interest 
arises  as  to  what  particular  material  it  is  which  is  found 
in  this  layer  of  the  solar  substance.  The  result  is  ex- 
tremely interesting  and  instructive.  It  has  been  shewn 
by  Dr.  Johnstone  Stoney  that  in  all  probability  the  mate- 
rial which  confers  on  the  Sun  its  beneficent  power  is  one 
which  is  found  in  great  abundance  on  the  Earth,  where 
it  fulfils  purposes  of  the  very  highest  importance. 

§  42.  Importance  of  Carbon.  There  is  no  known  metal 
and  perhaps  no  substance  whatever  which  has  so  high 
a  temperature  of  fusion  as  has  the  element  carton.  A 


44  A&Twiffmy 

filament  of  carbon  and  a  filament  of  that  element  only 
will  remain  nnfused  and  unbroken  when  heated  by  the 
electric  current  into  the  dazzling  brightness  necessary 
for  the  effective  illumination  of  an  incandescent  lamp. 
Modern  research  has  now  suggested  that  just  as  the  elec- 
trician has  to  employ  carbon  as  the  immediate  agent  in 
producing  the  brightest  artificial  lights  down  here,  so  the 
Sun  in  heaven  uses  precisely  the  same  element  as  the  im- 
mediate agent  in  the  production  of  its  transcendent  light 
and  heat.  Owing  to  the  extraordinary  fervour  which  pre- 
vails in  the  interior  parts  of  the  Sun  all  substances  there 
present  must  in  all  probability  be  not  only  melted,  but 
even  transformed  into  vapour.  In  the  presence  of  the 
intense  heat  of  the  inner  parts  of  the  great  luminary  even 
carbon  itself  does  not  continue  solid  or  liquid.  It  would 
seem  that  it  must  there  assume  the  vaporous  form  just 
as  copper  and  iron  and  other  substances  which  yield  more 
readily  to  the  fierce  heat  of  their  surroundings. 

The  buoyancy  of  carbon  vapour  is  one  of  its  character- 
istics. Its  vapours  consequently  ascend  in  the  solar 
atmosphere  to  a  higher  level  than  do  the  vapours  of  the 
other  elements.  We  can  understand  what  happens  to 
the  carbon  vapours  in  these  elevated  regions  by  the  analo- 
gous case  of  the  clouds  in  our  own  atmosphere.  It  is  true 
no  doubt  that  our  terrestrial  clouds  are  composed  of  mate- 
rial totally  different  from  that  which  constitutes  the  solar 
cloud.  It  is  of  course  beads  of  liquid  water  associated  in 
countless  myriads  which  form  the  clouds  we  know  so  well. 
As  the  buoyant  carbon  vapours  soar  through  the  Sun's 
atmosphere,  4hey  attain  an  elevation  where  the  fearful 
intensity  of  solar  heat  has  so  far  abated,  that  although 
nearly  all  other  elements  still  remain  in  the  gaseous  form 
there,  yet  the  exceptionally  refractory  carbon  begins  to 
return  to  the  liquid  or  the  solid  state.  Under  the  influence 
of  what  may  be  comparatively  called  a  chill,  the  carbon 


The  Sun  45 

vapour  collects  into  a  myriad  host  of  little  beads  of  liquid, 
or  it  may  be,  solid.  Each  of  these  infinitesimal  beads  of 
carbon  has  a  temperature  and  a  radiance  vastly  exceeding 
that  with  which  the  filament  glows  in  the  incandescent 
electric  lamp.  It  is  these  beads,  associated  in  clustering 
myriads,  that  constitute  the  glorious  solar  photospheric 
clouds.  The  entire  surface  of  our  luminary,  except  for  the 
occasional  interruption  of  spots,  is  coated  over  with  these 
incandescent  clouds,  of  Avhich  every  particle  is  intensely 
luminous.  We  need  thus  no  longer  wonder  at  that 
dazzling  brightness  which  even  across  the  awful  gulf  of 
nearly  ninety-three  millions  of  miles  produces  for  us  the 
indescribable  glory  of  daylight. 

§  43.  The  Quantity  of  Heat  emitted  by  the  Sun.  The 
heat  which  the  Sun  radiates  is  shot  forth  into  space  in 
every  direction  with  a  prodigality  which  seems  well-nigh 
inexhaustible.  The  share  of  Sun  heat  that  this  Earth  is 
able  to  capture  and  employ  forms  only  an  infinitesimal 
fraction  of  what  the  Sun  actually  pours  forth.  The  heat 
and  light  daily  lavished  by  that  orb  of  incomparable 
splendour  would  suffice  to  warm  and  illuminate,  quite  as 
efficiently  as  the  Earth  is  warmed  and  lighted,  more 
than  two  thousand  million  globes,  each  as  large  as  the 
Earth. 

Professor  Langley,  who  has  done  so  much  to  extend 
our  knowledge  of  the  great  orb  of  heaven,  has  suggested 
the  following  illustration  of  the  quantity  of  fuel  which 
\vould  be  required  to  maintain  the  Sun's  supply  of  heat,  if 
indeed  it  were  by  excessive  additions  of  fuel  that  the  Sun's 
heat  had  to  be  sustained.  Suppose  that  all  the  coal-fields 
which  underlie  England  and  Scotland,  America,  Australia, 
<  'hina,  and  wherever  else  coal  has  been  found  to  exist,  were 
compelled  to  yield  forth  every  combustible  particle  they 
contained ;  suppose  that  this  enormous  quantity  of  fuel. 
;i(li'(|ua1c  In  supply  the  wants  of  this  Earth  for  centimes, 


4G  Astronomy 

were  to  be  ignited,  vast  indeed  would  be  the  quantity  of 
heat  that  it  would  produce.  And  yet  it  is  perfectly  true 
that  a  conflagration  which  destroyed  every  particle  of  coal 
contained  in  this  Earth  would  not  generate  so  much  heat 
as  the  Sun  lavishes  abroad  in  the  tenth  part  of  every 
single  second. 


CHAPTER  III 
THE  APPARENT  MOTION  OF  THE  SUN 

§  44.  Apparent  Annual  Motion  of  the  Sun.  The  Sun 
shares  of  course  in  that  diurnal  motion  of  the  heavens  by 
\vhich  indeed  every  celestial  body  appears  to  perform  a 
complete  rotation  in  a  period  of  one  sidereal  day.  But  in 
addition  to  this  diurnal  movement,  common  to  all  the 
celestial  bodies,  the  Sun  has,  or  rather  it  should  be  said, 
appears  to  have,  a  certain  other  movement.  While  the 
stars  remain  constantly  fixed  in  the  same  position  rela- 
tively to  each  other,  the  place  of  the  Sun  on  the  celestial 
sphere  relatively  to  the  stars  is  in  a  state  of  incessant 
change.  No  doubt  it  is  not  possible  under  ordinary  cir- 
cumstances, at  least  without  the  help  of  the  telescope,  to 
see  stars  in  broad  daylight  in  the  vicinity  of  the  Sun. 
But  by  observing  the  fact  that  different  constellations  arc 
visible  at  different  times  of  the  year,  and  that  in  the 
course  of  a  year  these  changes  run  through  a  complete 
cycle,  it  may  be  seen  that  the  Sun  in  that  period  performs 
a.  complete  revolution  of  the  celestial  sphere  with  refer- 
ence to  the  stars. 

§  45.  The  Ecliptic.  If  we  could  imagine  the  stars  to  be 
visible  around  the  Sun  we  could  mark  out  the  track  which 
the  Sun  pursues  amongst  them.  By  means  of  the  meridian 
47 


48  Anti-otto  m  ;f 

circle  we  can  without  difficulty  determine  the  position  of 
the  Sun  with  regard  to  the  stars,  and  in  this  way  the 
actual  path  of  the  Sun,  or  rather  of  the  Sun's  centre,  on 
the  sphere  is  found  to  be  a  great  circle  to  which  the  name 
Ecliptic  has  been  given  since  it  is  only  when  the  Moon  is 
in  or  very  near  this  circle  that  it  is  possible  for  eclipses 
to  take  place. 

§  46.  The  Zodiac.  A  belt  of  the  heavens  extending  for 
8°  on  each  side  of  the  ecliptic  is  called  the  Zodiac.  Within 
this  belt  all  the  movements  of  the  Moon  and  of  those 
planets  which  were  known  to  the  ancients  are  contained. 
This  belt  is  divided  into  twelve  equal  portions,  each  30° 
long,  which  are  called  the  Signs  of  the  Zodiac.  The 
names  and  symbols  by  which  the  signs  are  denoted  are 
as  follows :  — 

T  Aries  SI  Leo  /  Sagittarius 

8  Taurus  "K  Virgo  VJ  Capricornus 

n  Gemini  =£=  Libra  z?  Aquarius 

<n>  Cancer  "I  Scorpio  X  Pisces 

The  first  of  these,  Aries,  extends  from  the  equinox  for 
30°  along  the  ecliptic,  the  second,  Taurus,  from  30°  to  60°, 
along  the  same  circle,  and  so  on. 

In  the  dawn  of  astronomy  when  the  zodiac  was  first 
mapped  out  the  constellations  which  bear  these  names 
coincided  with  the  several  signs,  hence  no  confusion  arose 
from  applying  the  names  indifferently  to  the  signs  and 
to  the  constellations.  A  slow  change  in  the  position  of 
the  equinox,  known  as  precession,  is,  however,  constantly 
carrying  the  equinox  backwards  through  the  Zodiac  so 
that  now  after  more  than  2000  years  it  has  shifted  its 
position  by  nearly  a  whole  sign.  The  consequence  is  that 
the  sign  Aries  lies  almost  wholly  in  the  constellation 
Pisces,  the  sign  Taurus  in  the  constellation  Aries  and 
similarly  for  the  others. 


'Tin-  Afifiu  rent  Motion   of  the  San 


41) 


§  47.  The  Obliquity  of  the  Ecliptic.  We  have  already 
explained  the  meaning  of  the  celestial  circle  which  we 
call  the  equator,  and  the  ecliptic  intersects  the  equator  in 
the  two  opposite  points  which  are  known  as  the  equinoxes. 
The  obliquity  of  the  ecliptic,  as  it  is  called,  is  the  angle 
between  this  great  circle  and  the  equator.  This  angle 
varies  somewhat  in  magnitude  but  its  movements  are 
confined  within  narrow  limits  and  its  value  at  present 
may  be  taken  to  be  23°  27'  8". 

§  48.  The  Vernal  Equinox  or  •  First  Point  of  Aries.'  At 
the  date  of  the  vernal  equinox,  that  is  on  the  21st  day 
of  March,  the  centre  of  the  Sun  is  at  one  of  the  points  in 
which  the  ecliptic  cuts  the  equator.  The  name  'Vernal 
Equinox '  which  is  strictly  applicable  to  the  moment  when 
the  Sun's  centre  is  at  this  point  is  often  extended  to  the 
point  itself.  This  point  is  also  called  '  The  First  Point 
of  Aries '  since  the  sign  Aries  is  measured  from  it  along 
the  ecliptic.  From  this  moment  the  Sun  ascends  above 
the  equator,  gradu- 
ally getting  higher 
and  higher  until  at 
the  period  known  as 
the  summer  solstice, 
which  is  at  present 
on  the  21st  day  of 
June,  the  Sun  attains 
its  maximum  height. 
From  this  onwards 
the  Sun  declines 
again  towards  the 
equator ;  it  reaches 
the  autumnal  equi- 
nox on  the  23rd  day 
of  September,  after 
which  the  centre  of  rig.  10.  Apparent  Annual  Path  of  tbo  Sun. 


50 

the  Sun  passes  below  the  equator,  gradually  increasing 
its  distance  from  that  circle,  until  the  winter  solstice  is 
reached  on  the  22nd  day  of  December,  when  the  Sun  is 
as  much  below  the  equator  as  it  was  above  it  at  the  sum- 
mer solstice,  its  distance  being  in  each  case  equal  to  the 
obliquity  of  the  ecliptic.  From  the  winter  solstice  the  cen- 
tre of  the  Sun  again  advances  towards  the  equator,  which 
it  gains  at  the  ensuing  vernal  equinox  on  the  21st  day  of 
March  when  the  phenomena  reappear  in  the  same  cycle. 

The  vernal  equinox  has  already  been  denned  as  one 
of  the  points  in  which  the  ecliptic  and  the  equator  in- 
tersect each  other.  It  has  been  found  convenient  for  the 
purposes  of  practical  astronomy  to  take  this  point  as  the 
point  of  reference  from  which  the  most  important  celestial 
measurements  are  made.  There  is  no  star  actually  situ- 
ated so  as  to  mark  the  vernal  equinox,  indeed  the  vernal 
equinox  itself  undergoes  slight  changes  in  its  situation. 
It  can  however  be  accurately  determined. 

Suppose  the  equinox  was  marked  by  a  visible  point, 
then  when  that  point  is  on  the  meridian  of  the  place  the 
sidereal  clock,  if  correct,  should  shew  0  hours  0  minutes 
0  seconds.  The  right  ascension  of  a  star  is  expressed  then 
by  the  sidereal  time  at  which  the  star  passes  the  meridian. 
If,  for  instance,  the  sidereal  time,  at  which  a  star  passes 
the  meridian  is  three  hours  and  twenty  minutes,  then 
what  we  mean  is  that  the  angle  between  the  great  circle 
from  the  vernal  equinox  to  the  Pole,  and  the  circle  from 
the  star  to  the  Pole  is  three  hours  and  twenty  minutes. 
\Ye  may,  if  we  please,  express  this  angle  as  the  intercept 
on  the  equator  made  between  these  two  great  circles. 
The  length  of  this  arc  if  expressed  in  time  would  be  also 
three  hours  and  twenty  minutes,  or  if  we  choose  to  turn 
it  into  angular  magnitude  at  the  rate  of  fifteen  degrees 
for  an  hour,  the  right  ascension  of  the  star  would  be  fifty 
degrees. 


Tli<-  Apparent  Notion  of  the  Sun 


r.l 


§  49.  Variation  in  the  length  of  the  Day.  The  move- 
ments of  the  Sim  will  explain  the  familiar  phenomena 
connected  with  its  varying  positions  in  the  sky  in  summer 
and  in  winter.  At  midsummer,  for  instance,  when  the  Sun 
is  above  the  equator  by  its  greatest  amount,  then  of  course 
at  noon  the  altitude  of  the  Sun  above  the  horizon  is  equal 
to  the  altitude  of  the  equatorial  point  on  the  meridian 
augmented  by  the  obliquity  of  the  ecliptic.  This  is  equal, 
in  fact,  to  the  co-latitude  of  the  place  (that  is  the  difference 
between  the  latitude  and  90°)  plus  the  obliquity.  In  this 
case  the  Sun  being  at  its  greatest  altitude  remains  for  a 
longer  time  above  the  horizon  than  at  any  other  period 
during  the  year  and  we  have  the  long  days  of  summer. 
Six  months  later  at  the  winter  solstice,  the  Sun  is  as 
much  below  the  equator  as  it  was  in  the  former  case 
above.  When  the  Sun  comes  to  the  meridian  its  altitude 
is  obtained  by  subtracting  the  obliquity  of  the  ecliptic 
from  the  co-latitude.  Its  meridian  altitude  is  then  a 
minimum,  and  therefore  the  time  it  remains  above  the 
horizon  is  less  than  at 
any  other  time  of  year. 
In  this  way  we  have  the 
short  days  of  winter. 

§  50.  Arctic  Day 
and  Night.  It  is  easy 
also  to  account  for  the 
fact  that  at  midsummer 
in  the  Arctic  regions 
the  Sun  does  not  set  at 
all.  For  as  the  altitude 
of  the  Pole  is  equal 
to  the  latitude  of  the 
place,  it  follows  that 
if  at  any  time  the  dis- 
tance of  the  Sun  from  Fig.  11.  Arctic  Day  and  Might. 


N.  Pole 


52  Axtroitom;/ 

the  Pole  be  not  greater  than  the  latitude  then  the  Sun  will 
not  set,  just  as  we  saw  to  be  the  case  for  the  circumpolar 
stars.  In  a  similar  manner,  we  can  explain  how  the  Sun 
does  not  rise  for  a  longer  or  shorter  period  about  the  time 
of  midwinter  in  the  Arctic  regions.  The  actual  number 
of  days  during  which  the  Son  is  invisible  depends  upon 
the  latitude  of  the  place. 

As  the  altitude  of  the  Pole  is  equal  to  the  latitude  (<£), 
the  polar  distance  of  the  south  point  of  the  horizon  is 
180°  —  <£.  If  we  denote  the  Sun's  declination  by  D,  then 
its  north  polar  distance  is  90°  -D.  Hence,  if  90°  -D  is 
greater  than  180°  —  (f>  the  Sun  will  be  below  the  horizon  even 
at  noon-day.  At  midwinter  the  Sun  is,  as  we  have  seen, 
23°  27'  below  the  equator  or  D=  -  23°  27'.  Therefore,  if 
we  put  90°  +  23°  27'  =  180°  -  <j>, 

we  have  <j>  =  90°  -  23°  27  '  =  66°  33' 

as  the  latitude  for  which  at  midwinter  the  Sun  at  noon  is 
just  on  the  horizon,  the  effect  of  refraction  being  omitted 
(§  54).  The  circle  on  the  Earth's  surface  corresponding 
to  the  latitude  66°  33'  is  called  the  Arctic  Circle. 

It  is  easy  to  shew  in  a  similar  wray  that  on  the  same 
circle  of  terrestrial  latitude  the  midsummer  sun  is  just  on 
the  northern  horizon  at  midnight.  For  the  altitude  of  the 
Pole  above  the  horizon  being  equal  to  the  latitude  of  the 
place,  and  the  Sun  being  by  hypothesis  on  the  horizon,  we 
have,  in  this  case,  the  latitude  (</>)  equal  to  the  Sun's  polar 
distance  (90°  -D).  Or 


But  at  midsummer  D=  +  23°  27',  hence, 

<£  =  <.H)0-230  27'  =  66°  33', 
that  is,  the  same  latitude  as  we  found  in  the  former  case. 

§  51.    To  find  the  length  of  the  Arctic  Night.      At  any 
place  within  the  Arctic  Circle  the  latitude  is  greater  than 


The  Apparent  Motion    of  the   &"//  53 

C.i;0  33 '.  and  for  any  given  place  it  is  easy  to  find  the  date 
on  which  the  Sun  ceases  to  rise  and  when  it  reappears. 
For  we  have  only  to  put, 

90°-y>  =  180°-^; 
whence  we  find 

/>  =  <£- 90°, 

which  gives  us  D,  the  declination  of  the  Sun,  and  we  can 
find  from  the  Xautical  Almanac  the  dates  on  which  the 
Sun's  declination  has  this  particular  value. 

Thus  if  we  take  <£  =  86°  14',  the  highest  latitude  ever 
yet  reached,  this  simple  equation  gives  us  U  —  —  3°  4G'. 
Prom  the  Nautical  Almanac  we  find  that  the  Sun's  dec- 
lination is  3°  46'  south  on  October  3  and  March  11,  so 
that  if  an  explorer  had  passed  the  winter  in  this  latitude 
he  would  never  have  seen  the  Sun  for  all  that  period  of 
nearly  six  months  from  October  3  to  March  11. 

Ina  similar  manner,  by  putting  90°  —  D  =  </>,  we  should 
find  the  date  on  which  the  Sun  ceases  to  set  even  at  mid- 
night for  any  latitude  <f>  within  the  Arctic  Circle  and 
could  deduce  the  length  of  the  continuous  arctic  da}'. 

§  52.  Apparent  Diameter  of  the  Sun.  These  circum- 
stances with  regard  to  the  movement  of  the  Sun  having 
been  established  we  may  next  consider  the  variations  in 
the  distance  of  the  Sun.  At  the  first  glance  it  appears 
that  the  Sun  is  sensibly  of  the  same  dimensions  from  one 
season  to  another.  And  as  the  apparent  dimensions  of  an 
object  depend  upon  its  distance  we  are  entitled  to  infer 
from  the  constancy  of  the  Sun's  apparent  diameter  that  its 
distance  remains  sensibly  uniform.  But  this  presumption 
has  to  be  modified  when  careful  measurements  are  made. 
With  the  help  of  certain  instruments  we  can  measure  the 
diameter  of  the  Sun  with  much  accuracy.  We  are  not  of 
course  referring  at  this  moment  to  the  measurement  of 
the  diameter  of  the  Sun  in  miles,  but  rather  to  the 
measurement  of  its  angular  diameter,  that  is  to  say  we 


54 


Astro  a 'nit  if 


mean  the  angle  which  the  diameter  of  the  Sun  subtends 
at  the  eye  of  the  observer.  By  careful  measurements  of 
this  kind  it  is  shewn  that  the  apparent  diameter  of  the 
Sun  is  not  constant,  but  that  it  runs  through  a  certain 
cycle  of  changes.  In  winter  time  the  apparent  diameter 
of  the  Sun  is  greater  than  it  is  in  summer. 

§  53.  Apparent  Track  of  the  Sun.  It  is,  however,  obvi- 
ous that  the  greater  the  distance  of  the  Sun  the  less  will  be 
its  apparent  angular  diameter.  In  fact  we  may  say  with  a 
considerable  degree  of  accuracy  that  the  distance  of  the 
Sun  and  its  apparent  angular  diameter  are  inversely 
proportional  to  one  another.  Consequently  from  our 
measurements  of  the  Sun's  diameter  we  are  able  to  obtain 
expressions  for  the  relative  values  of  the  Sun's  distance. 
Thus,  knowing  by  observation  the  diameter  of  the  Sun  on 
a  number  of  days  throughout  the  year,  we  can  set  down 
lines  proportional  to 
its  distances  from  the 
Earth  at  the  cor- 
responding epochs. 
With  the  meridian 
circle  we  can  deter- 
mine the  direction 
of  the  Sun  just  as 
we  can  that  of  a 
star.  If,  therefore, 
we  take  any  point 
(F)  to  represent  the 
Earth,  and  from  it 
draw  a  line  in  the 
direction  in  which 
the  Sun  appeared  on  any  day,  and  on  it  cut  off  a  part 
inversely  proportional  to  the  measured  angular  diameter  of 
the  Sun  on  that  day,  and  if  we  proceed  in  the  same  way 
for  the  other  days  we  shall  get  a  number  of  points  (A,  B,  C, 


Fig.  12.     Apparent  Track  of  the  Sun. 


The  Ajijxu-i-iif  Motion  of  the  8iui  55 

etc.)  representing  the  positions  occupied  by  the  Sun.  In 
this  way  we  are  able  to  form  a  representation  of  the  ap- 
parent track  of  the  Sun.  That  track  is  obviously  not  a 
circle  of  which  the  Earth  is  the  centre.  Its  form  can  how- 
ever be  entirely  accounted  for  by  the  supposition  that  the 
centre  of  the  Sun  in  every  position  lies  on  a  well-known 
curve  called  an  ellipse,  the  centre  of  the  Earth  being  at  a 
focus  of  the  ellipse.  An  ellipse  may  be  defined  as  the 
locus  of  a  point  such  that  the  sum  of  its  distances  from  two 
fixed  points  is  constant.  The  two  fixed  points  are  then 
the  foci  of  the  curve.  An  ellipse  may  be  very  easily 
drawn  in  the  following  way.  In  a  sheet  of  paper  fix  two 
drawing  pins  to  represent  the  foci.  Over  these  throw  a 
loop  of  thread.  If  then  a  pencil  be  drawn  round  so  that 
while  writing  on  the  paper  its  point  keeps  the  loop  of  thread 
tightly  stretched  it  will  describe  the  important  curve  which 
is  easily  seen  to  be  an  ellipse  according  to  the  definition 
just  given. 

§  54.  Effect  of  Refraction.  In  speaking  of  the  rising 
and  the  setting  of  the  Sun  it  is  always  necessary  to  remem- 
ber that  the  observed  phenomena  are  largely  modified  by 
the  effect  of  refraction.  "We  have  already  explained  how, 
owing  to  refraction,  the  apparent  place  of  every  object  is 
pushed  upwards  towards  the  zenith.  This  effect  is  par- 
ticularly marked  at  the  horizon.  And  in  fact  the  amount 
of  refraction  at  the  horizon  is  so  great  as  to  lift  the  Sun 
through  an  angle  greater  than  its  apparent  diameter.  It 
may  be  mentioned  that  the  apparent  diameter  of  the  Sun 
in  winter  is  32'  32"  and  in  midsummer  31'  32".  So  practi- 
cally we  may  take  the  average  value  of  the  Sun's  diameter 
at  about  thirty-two  minutes.  We  find  however  that  the 
horizontal  refraction  exceeds  thirty-three  minutes,  and 
hence  it  follows  that  even  after  the  Sun  has  completely 
set,  from  a  geometrical  point  of  view,  the  effect  of  refrac- 
tion is  such  as  to  make  the  whole  body  of  the  Sun  appear 


56 

still  above  the  horizon.  Since  at  the  eastern  horizon  this 
effect  of  refraction  tends  to  accelerate  the  Sun's  rising 
while  at  the  western  horizon  it  tends  to  retard  his  setting, 
we  see  that  from  sunrise  to  sunset  there  is  a  double  reason 
why  the  Sun  appears  to  be  longer  above  the  horizon  than 
it  would  have  been  if  the  atmosphere  had  not  possessed 
this  refractive  power. 

§  55.  Solar  Day.  We  have  already  explained  what 
is  meant  by  the  sidereal  day ;  it  is  the  time  required  by 
the  Earth  to  perform  one  rotation  on  its  axis.  As  a  mat- 
ter of  fact  this  is  measured  by  the  interval  between  two 
successive  transits  of  a  fixed  star.  If  however  we  take 
two  successive  transits  of  the  Sun,  the  interval  between 
them  is  not  the  same  as  the  interval  between  two  succes- 
sive transits  of  a  star.  For  the  Sun  is  moving  relatively 
to  the  stars.  If  the  Sun  and  a  star  came  on  the  meridian 
at  the  same  time  to-day  then  when  that  star  returned  to 
the  meridian  to-morrow  the  Sun  would  have  moved  four 
minutes  further  back,  so  that  four  minutes  more  would 
have  to  elapse  before  the  centre  of  the  Sun  crossed  the 
meridian.  If  we  take  this  observation  day  after  day,  we 
find  that  there  is  some  variation  in  the  interval  by  which 
the  Sun  moves  back,  this  variation  being  due  to  the  fact 
that  the  Sun's  apparent  movement  does  not  take  place 
uniformly.  Sidereal  time  would  not  be  adapted  for  the 
purposes  of  ordinary  life  for  we  must  regulate  our  hours 
by  the  Sun.  It  might  therefore  seem  natural  to  take  as 
our  day  the  interval  between  two  successive  passages  of 
the  Sun  across  the  meridian.  This  interval  is  however  not 
a  constant  one,  but  if  we  take  a  very  great  number  of  such 
intervals  between  tAvo  successive  transits  of  the  Sun  and 
if  we  take  the  average  of  them  all,  we  obtain  what  we 
call  the  mean  solar  day.  This  is  the  unit  that  is  em- 
ployed, for  all  ordinary  purposes,  in  the  measurement  of 
time.  Expressed  in  sidereal  time  the  mean  solar  day  is 


'/'//'•   AinHirmd  Motion   of  the  Sun  57 

24  hours  3  minutes  56-56  seconds.  This  mean  solar  day 
is  divided  into  24  hours,  each  hour  into  60  minutes, 
and  each  minute  into  60  seconds.  Owing  to  the  irregu- 
larity of  the  Sun's  movements  we  do  not  employ  the  Sun 
itself  when  thinking  of  our  measurement  of  time ;  we 
rather  introduce  the  conception  of  a  fictitious  Sun  which 
moves  uniformly  in  the  equator.  The  true  Sun  moves 
in  the  ecliptic  as  we  have  seen,  and  with  a  varying  speed ; 
this  fictitious  Sun  however  moves  in  such  a  way  that  on 
the  average  its  movement  coincides  with  that  of  the  true 
Sun.  When  this  fictitious  Sun,  or  rather  its  centre,  crosses 
the  meridian  then  a  clock  which  is  regulated  to  mean  time 
should  shew  0  hours,  0  minutes,  0  seconds. 

§  56.  Twilight.  The  atmosphere  has  another  effect 
on  the  distribution  of  sunlight  which  is  called  twilight. 
After  the  Sun  has  set  darkness  does  not  immediately  follow. 
There  is  still  a  certain  amount  of  light  known  as  twilight 
which  gradually  passes  away  as  the  night  comes  on.  This 
arises  from  rays  of  the  Sun  which  after  the  Sun  has  set 
traverse  the  higher  regions  of  the  atmosphere  and  there 
meet  with  the  particles  which  the  air  commonly  holds  in 
suspension.  Those  motes  that  are  seen  floating  in  a  sun- 
beam are  more  or  less  present  even  at  great  heights  in  the 
atmosphere,  and  it  is  easy  to  see  that  the  particles  which 
intercept  the  light  will  become  themselves  illuminated 
and  thus  shed  down  that  radiance  which  AVC  call  twilight. 
Twilight  however  ceases  when  the  Sun  has  reached  a  cer- 
tain distance  below  the  horizon.  That  distance  is  found 
to  be  about  18  degrees.  So  long  as  the  Sun  is  within  18 
degrees  of  the  horizon  some  of  its  light  will  reach  us  in 
this  manner.  We  can  thus  explain  the  circumstance  that 
at  midsummer  in  our  latitude  there  is  twilight  all  through 
the  night.  Take,  for  example,  a  latitude  of  53  degrees. 
Then  the  Pole  is  53  degrees  above  the  horizon.  On 
Midsummer  Day  the  distance  of  the  Sun  from  the  Pole 


58  Astronomy 

is  found  by  subtracting  the  obliquity  of  the  ecliptic  from 
1)0  degrees.  That  distance  is  66  degrees  33  minutes  (§  50). 
Hence  it  follows  that  at  midsummer  the  North  polar 
distance  of  the  Sun  must  be  66  degrees  33  minutes.  At 
midnight  the  Sun  is  of  course  below  the  horizon  and  will 
be  found  at  a  point  drawn  from  the  Pole  down  towards  the 
Xorth  and  continued  below  until  a  distance  of  66  degrees 
33  minutes  from  the  Pole  has  been  reached.  As  however 
the  latitude  is  53  degrees,  this  being  subtracted  from  the 
Sun's  polar  distance  shews  us  that  the  Sun  must  be  13 
degrees  33  minutes  below  the  horizon,  but  not  more.  We 
have  however  seen  that  there  is  twilight  whenever  the 
Sun  is  within  18  degrees  of  the  horizon,  and  hence  we 
see  that  at  the  latitude  we  have  chosen  there  is  twilight 
all  through  the  night  at  midsummer. 

§  57.  The  Sun's  Motion  only  Apparent.  Just  as  it 
was  found  that  the  diurnal  motion  of  the  heavens  was 
only  an  apparent  motion  and  had  indeed  to  be  explained 
by  the  supposition  that  it  was  the  Earth  itself  which 
rotated  on  its  axis  one  way  and  not  the  celestial  sphere 
which  rotated  the  opposite  way,  so  likewise  we  have  to  be 
prepared  for  a  widely  different  explanation  of  the  apparent 
movements  of  the  Sun  than  that  which  our  senses  appear 
to  suggest.  It  can  be  shewn  that  every  circumstance 
connected  with  the  apparent  revolution  of  the  Sun  can 
be  completely  accounted  for  by  supposing  that  the  Sun 
is  at  rest,  but  that  the  Earth  is  revolving  around  it. 
What  we  actually  seem  to  observe  is  an  apparent  displace- 
ment of  the  Sun  with  respect  to  the  stars.  But  a  moment's 
consideration  will  shew  that  such  displacements  do  not 
necessarily  require  the  Sun  to  be  in  motion,  they  could 
be  accounted  for  by  supposing  that  the  Earth  was  itself 
in  movement.  Is  it  not  plain  that,  if  the  Earth  were 
revolving  around  the  Sun,  an  observer  standing  on  the 
Earth  and  looking  towards  the  Sun  would  see  the  Sun 


The  Apparent  Motion  of  the  Suit  59 

projected  against  a  part  of  the  celestial  sphere  which  would 
be  continually  changing  ?  This  is  indeed  no  more  than 
what  is  actually  observed,  and  we  may  thus  account  for  the 
apparent  movement  of  the  Sun.  The  fact  is  the  observed 
phenomena  of  the  Sun's  movements  could  easily  be  ex- 
plained on  either  supposition,  and  when  we  reflect  that  the 
diameter  of  the  Sun  is  more  than  one  hundred  times  the 
diameter  of  the  Earth  and  that  the  Sun  is  consequently 
more  than  a  million  times  bigger  than  the  Earth,  it  seems 
much  more  reasonable  to  suppose  that  it  is  the  Earth  which 
moves  round  the  Sun  rather  than  the  Sun  which  moves 
round  the  Earth.  The  accuracy  of  this  presumption  is 
demonstrated  in  numberless  other  ways. 


CHAPTER   TV 

THE  Moox 

NEXT  to  the  Sun,  the  most  important  and  conspicuous 
of  the  celestial  bodies,  to  us  earth-dwellers,  is  the  Moon. 
This  position  of  distinction  is  not,  however,  due  to  any 
intrinsic  superiority  in  point  of  size  or  brilliance  on  the 
part  of  the  Moon  itself.  It  is  a  consequence  of  the  fact 
that  the  Moon  as  compared  with  all  other  celestial  bodies 
is  quite  near  to  us  —  is,  in  fact,  an  attendant  or  satellite 
of  the  Earth  revolving  around  the  Earth  at  a  distance  of 
less  than  a  quarter  of  a  million  of  miles  and  accompanying 
our  globe  in  its  ceaseless  revolutions  round  the  Sun. 

§  58.  The  Motion  of  the  Moon.  The  motion  of  the 
Moon  in  the  heavens  can  easily  be  detected  by  comparing 
its  position  from  hour  to  hour  with  regard  to  the  stars  near 
which  it  happens  to  pass.  After  a  brief  interval  it  will 
be  quite  plain  that  although  the  Moon  partakes  of  the 
general  motion  of  the  heavens,  rising,  that  is  to  say,  in 
the  east,  gradually  getting  higher  and  higher  in  the  sky 
till  it  reaches  the  meridian,  and  then  slowly  sinking  to  its 
setting  in  the  west,  yet  it  is  also  in  motion  with  regard  to 
the  stars. 

A  careful  watch  will  show  that  in  24  hours  the  Moon 
passes  over  about  13°  of  its  path,  and  that  in  a  period  of 


The  Moon  61 

27^  days  it  performs  a  complete  revolution  of  the  heavens. 
This  time  is  called  the  Sidereal  Period  of  the  Moon,  and  its 
accurate  value  is  27  days  7  hrs.  43  mins.  11-5  sees. 

§  59.  The  Orbit  of  the  Moon.  If  the  angular  diame- 
ter of  the  Moon  is  measured  carefully  in  various  positions 
then  it  is  possible  to  determine  the  path  in  which  it  moves 
around  the  Earth  in  a  way  similar  to  that  in  which  the 
apparent  orbit  of  the  Sun  is  found,  as  explained  in  §  53. 
It  is  thus  ascertained  that  the  Moon  describes  a  nearly 
circular  orbit  around  the  Earth,  its  true  form  being  an 
ellipse  of  small  eccentricity  of  which  the  centre  of  the 
Earth  occupies  the  focus.  The  plane  in  which  this  move- 
ment takes  place  is  nearly  the  same  as  that  in  which  the 
Earth  moves  around  the  Sun,  the  two  being  inclined  at  an 
angle  of  only  5°. 

§  60.  The  Phases.  We  are  now  in  a  position  to  ex- 
plain the  phases  of  the  Moon  with  which  all  are  familiar. 
The  Moon  being  a  dark  body  would  be  totally  invisible  if  it 
were  not  for  the  sunlight  reflected  from  its  surface.  The 
Sim  will,  of  course,  illuminate  only  half  of  the  Moon's 
surface  at  the  same  time  and  the  extent  of  the  lunar  phase 
depends  upon  the  amount  of  that  illuminated  hemisphere 
which  we  are  able  to  see. 

§  61.  Another  Moon  Period.  Although  the  Sidereal 
Period  of  the  Moon  is  no  more  than  27£  days,  yet,  as  is  well 
known,  the  interval  from  New  Moon  to  New  Moon  or  from 
Full  to  Full  is  29^  days.  To  explain  this  discrepancy  it  is 
only  necessary  to  remark  that  the  phases  depend  on  the 
position  of  the  Moon  relatively  to  the  Sun  and  that  in  the 
interval  between  one  Full  Moon  and  the  next  the  Sun  lias 
moved  forward  in  the  sky.  Thus,  if  at  the  time  of  Full 
Moon  we  note  the  position  of  the  Moon  with  regard  to  the 
stars  around  it.  then  after  an  interval  of  27^  days  the  Moon 
Avill  be  found  once  more  in  the  same  position,  but  in  the 
interval  the  Sun  has  moved  on,  and  it  requires  more  than 


62  Astronomy 

two  days  for  the  Moon  to  make  up  this  motion  so  as  once 
more  to  get  directly  opposite  to  the  Sun  in  the  only 
position  in  which  Full  Moon  can  occur. 

§  62.  Relation  between  the  Sidereal  and  the  Synodic 
Periods.  The  interval  from  New  to  New.  or  from 
Full  to  Full,  is  called  the  Synodic  Period  of  the  Moon, 
and  is  related  to  the  Sidereal  Period  and  the  length  of  the 
year  as  follows. 

If  S  be  the  number  of  days  in  the  Moon's  sidereal 

period  then  -  represents  the  fraction  of  a  complete  sidereal 

period  which  the  Moon  describes  in  one  day. 

In  the  same  way,  since  there  are  365£  days  in  the  year 
it  follows  that  in  one  day  the  Sun  performs  the  fraction 

7-^:7  of  a  complete  revolution  (here  we  refer  of  course  to 

the  apparent  revolution  of  the  Sun).  Hence  the  fraction 
of  a  complete  revolution  which  the  Moon  in  one  day  gains 

on  the  Sun  is  7,  — 


S     365J 
If  we  take  M  so  that 

1=  1--JL 
M     S     365£' 

then  the  Moon  gains  the  fraction  —  of  a  complete  revolu- 
tion on  the  Sun  each  day  and  consequently  in  M  days  it 
will  gain  a  complete  revolution.  That  is  to  say,  3/  is  the 
number  of  days  in  the  Synodic  Period.  In  this  way,  if 
S  is  taken  as  27£  days  we  find  that  3/  is  equal  to  29^ 
days. 

§  63.  Distance,  Size  and  Weight  of  the  Moon.  The 
mean  distance  of  the  Moon  from  the  Earth  is  found  to  be 
238,800  miles.  Its  mean  angular  diameter  is  31'  26"  which 
at  the  distance  of  238,800  miles  corresponds  to  2160  miles. 
It  is  thus  seen  that  the  Moon's  diameter  is  only  a  little 


The  Moon  63 

more  than  a  quarter  of  that  of  the  Earth.  It  would  take 
50  bodies  each  as  large  as  the  Moon  to  make  up  the  volume 
of  the  Earth.  Fifty  such  bodies  rolled  into  one  would  not, 


Fig.  13.     Comparative  Sizes  of  the  Earth  and  the  Moon. 

however,  weigh  as  much.  The  materials  of  which  the  Moon 
is  composed  being  on  the  average  lighter  than  those  of  the 
Earth,  more  than  80  Moons  would  be  required  to  outweigh 
our  globe. 

§  64.  Eclipses.  Since  the  Earth  is  an  opaque  body  it 
follows  that  all  the  rays  of  the  Sun  which  fall  upon  it  are 
stopped,  and  consequently  on  the  other  side  of  the  globe 
from  the  Sun  not  only  is  the  surface  itself  in  darkness  but 
there  is  a  vast  region  of  space  from  which  the  Earth,  like 
a  great  screen,  cuts  off  the  sunlight. 

If  we  imagine  a  cone  described  so  as  to  envelop  both 
the  Sun  and  the  Earth,  then  the  portion  of  space  occupied 
by  the  Earth's  shadow,  into  which  no  direct  ray  of  the  Sun 
can  penetrate,  is  the  part  of  that  cone  which  extends 
beyond  the  Earth  on  the  further  side  from  the  Sun. 

§  65.  Eclipses  of  the  Moon.  A  section  of  such  a  cone 
is  exhibited  diagrammatically  in  Fig.  14,  in  which  no 
attempt  has  been  made  to  preserve  the  proper  proportions 
l)ft \\-PPU  the  sizes  of  the  bodies  or  their  distances.  From 


64  Astronomy 

every  point  within  the  space  of  which  Ptt'  is  a  section  the 
Sun  is  wholly  invisible. 


Fig.  14.     Eclipses  of  the  Moon. 

But  there  is  another  cone  which  is  of  importance  in  the 
theory  of  eclipses.  This  is  a  cone  with  its  vertex  at  Q  and 
extending  to  envelop  the  Sun  in  one  direction  and  the 
Earth  in  the  other.  This  cone  is  represented  in  Fig.  14 
by  the  lines  T'Qr  and  TQr',  and  it  will  be  seen  that  from 
every  point  within  the  lightly  shaded  portion  of  this  cone 
the  Earth  cuts  off  more  or  less  of  the  light  of  the  Sun. 
This  cone  of  partial  obscuration  is  called  the  penumbra. 
We  must,  therefore,  think  of  the  Earth  in  its  motion 
round  the  Sun  as  being  always  accompanied  by  this  double 
cone  of  shadow  extending  out  behind  it  into  space. 

The  length  of  the  shadow  varies  a  little  from  time  to 
time  according  to  the  actual  distance  between  the  Earth 
and  the  Sun  but  it  can  easily  be  calculated  in  the  follow- 
ing way.  From  the  similarity  of  the  triangles  PTS  and 
PtE  we  have  the  proportion 

PE:PS  =  Et:NT, 

and  therefore 

PE :  PS  -  PE  =  Et  :ST-  Et. 


The   Mnnn  65 

But  PS  —  PE  is  SE,  i.e.  the  distance  of  the  Sun,  and 
therefore  we  find 

SE  .  Et          1 


PE 


ST-Et 


O/Tff 

Now  ^=-  is  the  ratio  which  the  diameter  of  the  Sun 
kit 

bears  to  the  diameter  of  the  Earth  and  is  equal  to  about 
108 ;  therefore  PE,  the  length  of  the  shadow,  is  about 
yiyth  of  the  distance  between  the  Earth  and  Sun,  or  on 
the  average  about  860,000  miles. 

§  66.  Lunar  Eclipses.  If  the  motion  of  the  Moon 
around  the  Earth  took  place  in  the  plane  of  the  ecliptic 
it  is  clear  that  the  Moon,  which  revolves  at  a  distance  of 
about  240,000  miles,  would  plunge  into  this  shadow  at 
each  revolution.  If  this  were  the  case,  therefore,  we 
should  have  a  lunar  eclipse  every  month  at  the  time  of 
Full  Moon. 

The  Moon's  orbit  is,  however,  inclined  at  an  angle  of 
5°  to  the  plane  of  the  ecliptic.  The  Moon  therefore  some- 
times passes  above  the  shadow  and  sometimes  belo\v.  If 
however  at  the  time  of  Full  Moon  our  satellite  should 
happen  to  be  very  near  the  ecliptic,  passing  from  one 
side  of  that  plane  to  the  other,  then  the  Moon  enters  the 
shadow  and  is  eclipsed.  If  the  lunar  globe  is  wholly 
immersed  in  the  shadow  then  it  is  said  to  be  totally 
eclipsed.  If  only  a  portion  of  the  globe  is  obscured  then 
it  is  said  to  be  partially  eclipsed. 

$  67. .  Colour  of  the  Moon  in  an  Eclipse.  It  should 
be  here  pointed  out  that  if  the  Earth  were  simply  a 
solid  globe  then  no  light  whatever  could  penetrate  into 
the  Earth's  shadow,  and  the  Moon  in  a  total  eclipse 
would  always  be  quite  invisible.  The  Earth  is,  however, 
as  we  know,  surrounded  by  a  vast  atmosphere  and  this 
has  the  effect  of  bending  the  rays  which  strike  obliquely 


66  Astronomy 

upon  it  and  refracting  some  of  them  into  the  cone  of 
shadow,  which  is  accordingly  not  absolutely  dark.  In 
their  passage  through  the  atmosphere  these  rays  become 
tinged  to  some  extent  with  a  reddish  colour,  and  the 
consequence  is  that  the  Moon  in  a  total  eclipse  is  not 
wholly  lost  to  sight  but  can  generally  be  seen  shining 
with  a  dull  copper-coloured  light. 

§  68.  Solar  Eclipses.  In  general  the  inclination  of 
the  Moon's  orbit  carries  the  '  New  Moon '  above  or  below 
the  Sun  as  seen  from  the  Earth.  But  if  at  the  time  of 
New  Moon  our  satellite  happens  to  be  passing  from  the 
North  to  the  South  of  the  ecliptic,  or  from  the  South  to 
the  North,  it  may  so  happen  that  the  Moon  will  appear 
directly  in  front  of  the  Sun  and  an  eclipse  of  the  latter 
body  will  take  place. 

The  Moon,  of  course,  casts  behind  it  a  shadow  in  just 
the  same  way  as  we  have  already  seen  the  Earth  to  do. 
We  can  calculate  the  length  of  this  shadow,  just  as  we  did 


Moon 


Fig.  15.     Total  Eclipse  of  Sun. 


that  of  the  Earth,  from  the  known  relation  between  the 
sizes  of  the  Sun  and  Moon.  Thus  it  is  found  that  the 
average  length  of  the  shadow  of  the  Moon  when  it  is  placed 
directly  between  the  Sun  and  the  Earth  is  232,000  miles. 


The  Moon  67 

The  average  distance  of  the  Moon  from  the  centre  of  the 
Earth  is  238,800  miles  and  the  semi-diameter  of  the  Earth 
is  3959  miles,  so  that  the  average  distance  from  the  Earth's 
surface  to  the  Moon  is  234,800  miles.  Thus  we  see  that 
the  point  of  the  shadow  would  fall  short  of  the  Earth's 
surface  under  average  circumstances  by  about  2900  miles. 
But  since  both  the  length  of  the  shadow  and  the  distance 
of  the  Moon  vary  considerably  it  frequently  happens  that 
the  shadow  is  long  enough  to  extend  to  the  Earth's  sur- 
face, and  in  that  case  it  is  clear  that  from  all  points  within 
the  patch  of  darkness  where  the  shadow  falls  the  Sun 
would  be  invisible  or  totally  eclipsed,  while  places  situated 
within  the  section  of  the  Moon's  penumbra  enjoy  the 
phenomenon  of  a  partial  eclipse. 

§  69.  Annular  Eclipse.  If  at  the  time  when  an  eclipse 
occurs  the  shadow  does  not  reach  as  far  as  the  surface 
then  it  is  clear  that  an  observer  situated  as  at  a  in  Fig. 
16,  will  see  the  Moon  projected  against  the  Sun's  disc 
while  a  narrow  ring  of  light  will  be  visible  all  round  it. 
This  is  known  as  an  '  Annular  Eclipse  of  the  Sun.' 


Moon 


.Fig.  10.     Annular  Eclipse  of  Sun. 

§  70.  Total  Eclipse  of  Sun.  Of  the  various  kinds  of 
eclipses  which  may  occur  by  far  the  most  important  is 
that  known  as  a  total  eclipse  of  the  Sun ;  for  when  this 


68  Astronomy 

takes  place,  and  the  direct  light  from  the  bright  photo- 
sphere is  cut  off,  then,  as  has  been  mentioned  in  the  last 
chapter,  we  are  able  to  see  the  faint  appendages  of  the 
Sun  outside  its  photosphere  which  are  known  as  the 
prominences  and  the  Corona,  and  which  at  other  times 
and  under  ordinary  circumstances  are  quite  invisible. 

§  71.  The  Constant  Face  of  the  Moon.  The  most  obvi- 
ous fact  with  regard  to  the  appearance  of  our  satellite 
arises  from  the  circumstance  that  it  rotates  on  its  axis  in 
the  same  time  that  it  takes  to  make  a  complete  revolu- 
tion around  the  Earth.  This  is,  in  fact,  merely  another 
way  of  stating  that  the  Moon  constantly  turns  the  same 
face  towards  the  Earth. 

§  72.  The  Librations.  If  the  Moon  moved  with  per- 
fect uniformity  in  its  orbit  and  if  the  axis  around  which 
it  rotates  were  perpendicular  to  the  plane  in  which  its 
orbit  lies  then  we  should  never  catch  a  glimpse  of  more 
than  exactly  one-half  of  the  lunar  surface.  But  in  conse- 
quence of  the  elliptic  form  of  its  orbit  the  Moon  moves 
with  different  speeds  at  different  parts  of  its  track.  The 
motion  on  its  axis  is,  however,  uniform  and  hence  we  can 
sometimes  see  a  little  round  the  eastern  limb  and  some- 
times a  little  round  the  western.  Also  when  the  Moon  is 
in  that  part  of  its  orbit  where  the  northern  end  of  its  axis 
inclines  towards  the  Earth  then  we  are  enabled  to  see  for 
a  short  distance  beyond  the  North  Pole,  and  when  the 
southern  end  leans  in  our  direction  we  see  a  little  beyond 
the  Southern  Pole.  These  phenomena  are  called  the 
Librations  of  the  Moon.  The  glimpse  of  the  other  side 
is  however  very  partial ;  only  9  per  cent,  of  the  total  sur- 
face is  thus  occasionally  revealed,  and  even  then,  it  is 
presented  so  obliquely  to  our  view  that  but  little  is  added 
to  our  knowledge  of  the  lunar  features. 

§  73.  Libration  of  the  Moon.  The  first  of  these  Libra- 
tions is  illustrated  in  an  exaggerated  form  in  Fig.  17. 


j.  17.     Libration  of  the  Moon  in 
Longitude. 


If  ABCD  be  the  orbit  of  the  Mooji  around  the  Earth  at  E, 
then  A  represents  the 
Moon's  Perigee  and  C 
the  Apogee.  Also,  on  a 
date  half-way  between 
the  two  positions  A  and 
C  the  Moon's  position 
will  be  as  represented 
at  B.  Now  if  m-i  repre- 
sent a  mountain  sup- 
posed to  be  just  at  the 
centre  of  the  visible 
hemisphere  at  A ;  when 
the  Moon  reaches  B, 
one-quarter  of  the  whole  period  having  elapsed,  the 
mountain  will  have  made  exactly  one-quarter  of  a  rotation 
around  the  axis  and  will  be  situated  as  at  m2,  and  will  no 
longer  occupy  the  centre  as  seen  from  E.  When  the  Moon 
gets  to  C,  having  made  half  a  revolution,  the  mountain 
will  be  in  the  position  ms  and  will  once  more  be  centrally 
placed.  But  when  three-quarters  of  the  Moon's  period  has 
elapsed  the  Moon  will  have  reached  D  while  the  mountain 
having  made  three-quarters  of  a  rotation  will  be  at  mt 
and  will  again  appear  disturbed  from  its  central  position. 
This  is  the  Libration  in  Longitude. 

Of  what  may  exist  on  the  further  side  of  the  Mooil 
we  have  absolutely  no  direct  knowledge.  But  there  is  no 
reason  to  think  that  the  features  it  would  display  are 
essentially  different  from  those  on  the  side  of  the  Moon 
which  is  turned  towards  us.  Ever  since  the  invention  of 
the  telescope  the  hemisphere  of  our  satellite  which  is 
presented  for  study  has  been  examined  so  closely  that  it 
may  be  asserted  that  there  is  no  marking  on  its  surface 
as  large  as  Hyde  Park  which  has  not  been  drawn  and 
mapped  and  in  most  cases  even  had  a  name  given  to  it. 


70 

§  74.  Lunar  Craters.  When  viewed  through  a  tele- 
scope the  surface  of  our  satellite  is  found  to  be  marked 
in  all  directions  by  craters  or  rings,  planes,  mountain 
chains,  and  large  dark  patches.  The  accompanying 
figure  represents  a  photograph  of  the  Moon  when  full 
and  shews  many  characteristic  objects. 

Among  the  principal  volcanic  remains  which. constitute 
perhaps  the  most  interesting  features  on  the  Moon's  surf  ace 
we  must  notice  the  great  craters  Tycho  and  Copernicus, 
which  are  specially  characterised  by  the  systems  of  streaks 
which  radiate  from  them.  Tycho,  indeed,  illustrates  the 
most  perfect  type  of  lunar  mountain,  although  in  some 
phases  of  illumination  when  the  sunshine  falls  upon  it 
obliquely,  it  is  difficult  to  distinguish  this  particular  crater 
among  other  less  important  craters  with  which  the  surface 
in  its  neighbourhood  is  pitted.  As  the  Sun  rises  on  Tycho, 
however,  the  preeminence  of  this  great  crater  becomes 
unquestioned,  and  then  the  magnificent  series  of  bright 
streaks  appear  which  radiate  from  it  in  all  directions  and 
form  that  very  striking  system  that  can  so  easily  be 
observed  in  a  telescopic  view  of  the  Full  Moon.  The 
enormous  cavity  in  the  centre  of  this  mountain  is  fifty- 
four  miles  across,  while  its  depth  is  more  than  three  miles 
below  the  summit  of  the  ring.  In  the  centre  a  rugged 
peak  ascends  to  a  height  of  about  one  mile.  The  ring  is 
composed  of  four  tiers  of  terraces  on  the  inner  side  of  its 
slope,  one  above  the  other,  of  an  extremely  steep  and 
rugged  nature. 

Close  to  the  eastern  limb  of  the  Moon  is  seen  the  well- 
marked  plane,  Grimaldi.  It  is  remarkable  for  its  dark 
colour,  apparently  due  to  some  peculiarity  of  the  soil  in  the 
interior.  Nearer  to  the  South  Pole,  which  is  the  upper 
pole  in  the  picture,  as  an  astronomical  telescope  always 
inverts,  lies  another  great  plane  surrounded  by  a  moun- 
tain rampart  very  much  foreshortened,  which  is  known 


The  Moon  71 

by  the  name  of  Schickard.  The  mighty  wall  which  encloses 
it  is  more  than  two  miles  high  in  some  parts  and  more 
than  four  hundred  and  sixty  miles  in  circumference.  A 
ring  so  vast  as  this  is  large  even  in  comparison  with  the 
Moon's  diameter,  and  consequently,  a  spectator  standing 
in  the  centre  might  well  imagine  himself  on  a  boundless 
plain,  since  owing  to  the  rapid  rounding  of  the  Moon's 
surface,  the  ring  which  encompassed  him  would  be  wholly 
out  of  sight  beyond  his  horizon. 

Another  interesting  object  is  the  ring  known  as  Plato. 
Its  great  rampart  is  sixty  miles  in  diameter.  On  the  floor 
are  found  several  small  crater-rings,  and  the  surface  of  the 
floor  is  variously  coloured  in  irregular  patches.  At  the 
south-eastern  end  a  large  mass  of  the  ring  has  fallen  down, 
forming  a  gigantic  landslip.  The  surface  of  the  Moon  has 
been  studied  so  carefully  that  a  volume  would  be  required 
to  describe  the  objects  which  are  known  and  of  which  we 
have  mentioned  a  few. 

We  naturally  ask  how  these  appearances  on  the  Moon 
are  to  be  accounted  for.  Formations  of  the  same  general 
type  are  found  scattered  in  the  greatest  profusion  all  over 
its  surface.  In  these  features,  we  find  a  nearly  circular 
ring  or  rampart,  the  interior  of  which  is  generally  depressed 
below  the  average  level  of  the  Moon  with  or  without,  as 
the  case  may  be,  a  central  peak.  We  are  not  without 
some  terrestrial  analogies.  There  are  certain  regions  of 
the  Earth  which  if  seen  from  above  under  certain  illu- 
mination, would  present  appearances  very  similar  to  those 
presented  by  these  obj ects  in  the  Moon .  In  the  neighbour- 
hood of  Vesuvius,  in  that  remarkable  volcanic  district,  in 
Auvergne  near  the  Puy  de  Dome,  and  in  certain  lava 
formations  in  the  Sandwich  Islands,  we  are  reminded 
forcibly  of  the  features  of  the  Moon.  We  are  thus  led  to 
attribute  the  numberless  pits  and  cones  which  dot  the 
face  of  our  satellite  to  volcanic  action.  We  have,  it  must 


72  Astronomy 

be  admitted,  notwithstanding  the  greater  scale  of  our 
Earth,  nothing  which  can  compare  with  the  gigantic  lunar 
Tycho  with  its  diameter  of  fifty  miles,  and  its  ring  of 
seventeen  thousand  feet  high.  As,  however,  we  find  lunar 
craters  formed  on  exactly  similar  lines,  varying  in  size  from 
the  smallest  spots  we  can  clearly  see,  up  to  enormous  ob- 
jects like  those  just  mentioned,  we  are  naturally  led  to 
explain  the  larger  objects  as  merely  the  result  of  extreme 
exertions  of  a  similar  character  to  those  which  produce  the 
smaller  objects. 

§  75.  Surface  Characteristics.  The  surface  of  the  Moon 
is  of  such  a  character  that  it  would  be  impossible  for  a 
traveller  placed  upon  it  to  make  much  progress  in  explora- 
tion without  encountering  tremendous  difficulties  owing  to 
the  peculiar  nature  of  the  lunar  country.  The  surface  of 
our  satellite  indeed  displays  deserts  far  more  worthy  of  the 
name  than  any  Saharas  which  our  globe  can  produce.  We 
need  not,  however,  expect  to  find  in  a  lunar  desert  the 
abundant  sand  which  often  characterises  deserts  on  the 
Earth.  The  Moon  seems  to  be  a  vast  waste  not  so  much 
of  sand  as  of  rock.  Over  the  greater  part  of  the  lunar 
globe  the  rocky  surface  is  so  rugged  and  mountainous  that 
but  few  regions  on  this  Earth  would  bear  comparison  with 
it.  But  a  lunar  traveller  would  be  continually  beset  with 
obstacles  of  a  much  more  troublesome  nature  than  those 
which  confront  the  Alpine  climber  when  he  is  scrambling 
over  the  rocks  in  Switzerland.  The  lunar  surface  is  largely 
cumbered  with  irregular  masses  of  rock  and  there  would  be 
no  such  facilities  for  getting  over  them  as  are  often  found 
on  this  Earth.  The  sharp  angles  of  the  lunar  rocks  have 
never  been  rounded  off  by  the  action  of  air  and  water, 
those  two  agents  which  have  been  of  such  conspicuous 
importance  in  the  sculpture  of  our  own  globe. 

There  is  also  another  class  of  difficulty  which  would 
greatly  embarrass  a  traveller  who  set  out  to  explore  the 


The  Moon  73 

Moon's  surface.  He  would  frequently  find  his  progress 
intercepted  by  a  deep  and  wide  crack  of  an  utterly  impass- 
able character.  Nor  need  he  in  general  hope  to  accomplish 
his  end  by  getting  round  the  extremity  of  such  a  fissure. 
Such  fissures  often  extend  for  hundreds  of  miles.  They 
are  sometimes  intersected  by  other  similar  chasms,  and 
these  abysses  are  usually  so  deep  that  from  our  point  of 
view  we  have  never  been  able  to  see  to  the  bottom  of 
them.  Owing  to  the  distance  of  the  Moon  a  fissure  has 
to  be  not  less  than  a  mile  in  width  to  be  distinctly 
visible  in  our  telescopes.  We  can  in  fact  only  observe 
those  rents  which  are  specially  prominent  in  the  lunar 
surface. 

§  76.  Extinct  Volcanoes.  Though  I  have  spoken  of 
the  mighty  Copernicus  as  a  volcano,  yet  it  must  be  borne 
in  mind  that  this  object  as  well  as  the  hundreds  of  other 
similar  features  on  the  Moon  are  not  to  be  regarded  as 
volcanoes  in  the  active  sense  of  the  word.  Our  satellite 
has  been  studied  most  carefully  ever  since  the  telescope 
was  invented,  but  no  observer  has  ever  yet  beheld  any 
disturbance  on  its  surface  which  could  be  interpreted  as 
a  volcanic  eruption  in  actual  progress.  The  Moon  was  no 
doubt  once  the  seat  of  volcanic  outbreaks  of  tremendous 
intensity,  but  those  days  have  long  since  passed.  All  we 
now  see  are  the  remains  of  volcanoes  which  have  been 
certainly  still  for  hundreds  of  years  and  probably  for 
thousands,  or  for  aught  we  can  tell  for  uncounted  millions 
of  years. 

§  77.  Sharpness  of  Lunar  Features.  Notwithstanding 
the  antiquity  of  the  lunar  features  the  observer  who  is 
privileged  to  look  at  our  satellite  through  a  good  telescope 
will  be  struck  with  the  wonderful  sharpness  and  definite- 
ness  of  outline  which  their  details  exhibit.  This  appear- 
ance of  freshness  has  to  be  accounted  for  and  we  are  able 
to  give  the  explanation.  The  avalanches  which  thunder 


74  Astronomy 

down  a  Swiss  mountain  are  in  one  way  or  another  due  to 
the  action  of  water  which  has  been  frozen.  It  may  be 
that  the  water  which  has  penetrated  in-to  a  crack  in  the 
rock,  expands  in  the  act  of  passing  into  ice.  Thus  great 
fragments  of  rock  are  loosened,  and  such  fragments, 
accompanied  it  may  be  by  great  volumes  of  snow,  break 
away  and  are  hurried  down  in  an  avalanche.  This  action, 
with  which  every  one  who  has  visited  the  Alps  is  familiar, 
is  incessantly  going  forward  in  almost  all  mountainous 
regions.  The  wearing  influence  of  water  in  its  various 
forms  is  ever  tending  to  crumble  away  the  rocks  and 
mountains  and  thus  to  reduce  and  lessen  the  irregularities 
on  the  Earth's  surface.  In  the  course  of  ages  such  opera- 
tions of  water  are  constantly  effecting  the  most  remarkable 
transformation  of  the  features  on  the  surface  of  the  land. 
There  can  hardly  be  a  doubt  that  if  the  mighty  crater 
Copernicus  had  been  situated  on  this  Earth,  instead  of  be- 
ing, as  it  is,  on  the  Moon,  the  incessant  operation  of  air 
and  water  would  long  ago  have  modified  the  aspect  of  the 
crater  from  that  which  it  still  continues  to  wear  under  the 
serene  conditions  under  which  it  is  placed  on  our  satellite. 

§  78.  Absence  of  Water.  Our  telescopes  shew  dis- 
tinctly that  on  the  lunar  world  are  neither  seas  nor  ocean. 
They  do  not  indicate  lakes  or  rivers,  they  do  not  even 
present  clouds  or  mists  such  as  would  necessarily  arise  if 
any  water  were  present.  We  have  never  seen  a  lunar 
mountain  peak  crowned  with  mist  and  no  wisp  of  vapour 
has  ever  been  detected  in  a  lunar  valley.  Thus  we  con- 
clude that  the  main  agent  for  wearing  down  our  mountains 
is  absent  from  our  satellite.  And  hence  we  need  feel  but 
little  surprise  that  notwithstanding  the  unknown  ages 
which  seem  to  have  elapsed  since  Copernicus  was  actually 
in  activity,  it  should  still  present  to  us  with  all  its  sharp- 
ness complete  the  grand  outlines  of  a  primeval  volcano. 

§  79.    Lunar   Atmosphere.      The    extraordinary  clear- 


The  Moon  75 

ness  with  which  the  telescopic  observer  is  enabled  to  scru- 
tinise the  features  of  the  Moon  is  largely  due  to  the  fact 
that,  unlike  our  Earth  in  this  respect,  our  satellite  is  sur- 
rounded by  no  appreciable  atmosphere.  This  Earth  of  ours 
is  closely  wrapped  around  with  a  coat  of  air  gradually 
decreasing  in  density  from  the  bottom  where  we  live  up 
to  about  two  hundred  miles  over  our  heads,  where  the 
attenuated  atmosphere  is  merging  into  open  space.  It  has 
sometimes  been  thought  that  in  the  lunar  valleys  there 
may  be  slight  traces  of  some  gaseous  body.  But  in  any 
case  such  covering  can  be  no  more  than  the  merest  fraction 
of  the  bounteous  atmosphere  which  our  Earth  enjoys. 

§  80.  History  of  the  Earth- Moon  System.  I  do  not 
think  there  is  any  chapter  in  modern  science  more  re- 
markable than  the  history  of  the  Moon  which  I  here  pro- 
pose to  describe.  Modern  research  has,  however,  conducted 
us  to  a  glimpse  of  what  has  taken  place  at  an  extremely 
early  period  of  the  Earth-Moon  history  —  the  theory  to 
which  I  now  refer  has  been  largely  due  to  the  researches 
of  Professor  G.  H.  Darwin  of  Cambridge. 

§  81.  The  Tides.  Our  argument  proceeds  from  a  sim- 
ple and  well-known  matter.  Every  one  who  has  ever  been 
on  the  sea-shore  knows  that  daily  ebb  and  flow  of  the  waters 
which  we  call  the  tides.  Long  before  the  true  nature  of 
the  forces  by  which  the  Moon  acts  upon  the  sea  was 
understood  it  had  become  certainly  known  that  there  was 
a  connexion  between  the  tides  and  the  Moon.  Indeed 
the  daily  observations  of  a  fisherman  or  of  any  one  whose 
business  was  concerned  with  the  great  deep  would  have 
taught  him  that  the  time  of  high  water,  at  the  particular 
part  of  the  coast  where  his  business  lay,  and  the  time  of 
Full  Moon  had  a  certain  definite  relation  to  each  other. 
The  fisherman  might  not  — he  certainly  did  not  —  under- 
stand the  precise  influence  of  the  Moon  upon  the  tides. 
If,  however,  he  had  noticed,  as  he  would  be  likely  to  do, 


76  Astronomy 

that  whenever  the  Moon  was  full  the  tide  was  high  at  ten 
o'clock  in  the  morning,  as  it  would  be  in  certain  ports,  it 
would  be  perfectly  obvious  to  him  that  the  Moon  had  some 
relation  to  this  ebbing  and  flowing  of  the  ocean.  The  time 
of  high  water  being  of  such  importance  to  the  daily  avoca- 
tions of  the  fisherman,  the  fact  that  when  the  Moon  was 
full  the  hour  of  high  water  was  always  the  same  at  his 
particular  port  would  hardly  have  escaped  his  notice. 

§  82.  Work  done  by  the  Tides.  As  the  tides  course 
backwards  and  forwards,  sweeping  to  and  fro  vast  volumes 
of  water,  it  is  obvious  that  the  tides  must  be  doing  work. 
In  fact,  in  some  places  the  tides  have  been  forced  to  do 
useful  work.  If  the  rising  water  be  impounded  in  a  large 
reservoir  it  can  be  made  to  turn  a  water-wheel  as  it  enters, 
while  as  the  reservoir  empties  itself  a  few  hours  later 
another  current  is  produced  which  can  be  utilised  in  a 
similar  manner.  Thus  we  can  produce  a  tidal  mill.  It 
may  be  quite  true  that  it  is  not  often  possible  to  employ 
the  direct  power  of  the  tides  in  an  economical  manner. 
It  is,  however,  for  our  purpose  merely  necessary  to  note 
that  day  after  day,  week  after  week,  year  after  year,  the 
tides  must  be  incessantly  doing  work  of  some  kind  or  other. 

§  83.  Earth's  Energy  of  Rotation.  Every  practical 
man  knows  that  work  can  only  be  accomplished  by  the 
expenditure  of  a  precisely  equivalent  amount  of  what  is 
known  as  energy.  He  knows  also  that  there  is  in  Nature 
no  such  operation  as  the  creation  of  energy.  It  is  just  as 
impossible  to  create  out  of  nothing  the  energy  which  would 
lift  an  ounce  weight  a  single  inch  as  it  would  be  to  create 
a  loaf  of  bread  out  of  nothing.  If  therefore  the  tides  are 
doing  work,  and  we  have  seen  that  they  undoubtedly  are 
so  employed,  it  follows  that  there  must  be  some  source  of 
energy  on  which  the  tides  are  able  to  draw.  There  is  only 
one  possible  source  for  the  energy  necessary  to  sustain  the 
tides.  The  Earth  may  be  regarded  as  a  mighty  fly-wheel 


The  Moon  77 

which  contains  a  prodigious  store  of  energy.  That  energy 
is  however  never  added  to,  for  there  is  no  agency  to  impart 
fresh  rotation  to  the  Earth.  If  no  energy  were  withdrawn 
from  the  Earth's  rotation  then  the  globe  would  continue 
for  ever  to  spin  round  its  axis  once  every  twenty-four 
hours.  As  the  tides  need  energy  to  get  through  their 
work,  they  abstract  what  they  require  from  the  store  which 
they  find  at  hand  in  the  rotation  of  the  Earth.  It  must 
indeed  be  carefully  understood  that  although  it  is  the 
attraction  of  the  Moon  drawing  the  water  towards  it  on 
the  one  side,  and  drawing  the  Earth  away  from  the  water 
on  the  other  side,  which  produces  the  tides,  yet  it  is  not 
the  Moon  which  supplies  the  energy  necessary  for  the  tides 
to  do  their  work.  Indeed  a  little  reflection  will  shew  that 
if  it  were  not  for  the  rotation  of  the  Earth  relatively  to  the 
Moon  there  would  be  no  rising  and  falling  of  the  tide. 
I  mean,  of  course,  that  if  the  Earth  and  the  Moon  rotated 
as  one  piece  then  the  high  tide  would  always  be  in  the 
same  place  on  the  Earth,  there  would  be  no  ebbing  and 
flowing,  and  consequently  there  would  be  no  consumption 
of  energy  by  the  tidal  movements.  It  is  the  rotation  of 
the  Earth  which  causes  the  Earth,  so  to  speak,  to  move 
against  the  tides  and  it  is  to  overcome  the  resistance  thus 
arising  that  the  perennial  supply  of  energy  is  demanded. 
This  withdrawal  of  energy  from  the  Earth  is  incessantly 
taking  place  along  almost  every  coast.  From  day  to  day5 
from  century  to  century,  energy  is  daily  being  with- 
drawn and  daily  being  expended  but  energy  is  never 
again  restored  to  the  Earth.  The  consequence  is  inevi- 
table. The  quantitj^  of  energy  due  to  the  rotation  of  the 
Earth  must  be  gradually  declining.  The  result  at  which 
we  have  arrived  involves  the  practical  consideration  that 
the  operation  of  the  tide  must  be  gradually  reducing  the 
speed  with  which  the  Earth  rotates.  The  tides  must,  in 
fact,  be  increasing  the  length  of  the  day. 


78  Astronomy 

This  is  indeed  a  notable  consequence  of  those  tides 
which  ripple  to  and  fro  on  our  shores  and  which  flow  in 
and  flow  out  of  our  estuaries.  Owing  to  these  tides  to-day 
is  longer  than  yesterday,  and  yesterday  longer  than  the 
day  before.  We  must  however  admit  that  the  change  pro- 
duced is  not  very  appreciable  when  only  moderate  periods 
of  time  are  considered.  Indeed  the  alteration  in  the 
length  of  the  day  from  this  cause  amounts  to  no  more 
than  a  fraction  of  a  second  in  a  period  of  a  thousand  years. 
Even  within  the  lapse  of  historic  time  there  is  no 
recognisable  change  in  the  length  of  the  day  attributable 
to  the  action  of  the  tide.  But  the  importance  of  our 
argument  is  hardly  affected  by  the  circumstance  that  the 
rate  at  which  the  day  is  lengthened  is  a  very  slow  one. 
The  point  which  is  really  significant  to  notice  is  that  this 
change  is  incessantly  taking  place  and  that  it  invariably 
tends  to  increase  the  length  of  the  day.  It  is  this  latter 
circumstance  which  gives  to  the  doctrine  its  great  im- 
portance as  a  factor  in  the  development  of  the  Earth-Moon 
system.  Astronomers  are  accustomed  to  investigate  move- 
ments in  the  celestial  bodies  which  advance  for  vast  periods 
in  one  direction  and  then  for  equally  long  periods  become 
reversed.  Such  movements  as  these  are,  however,  not  of 
the  kind  which  produce  really  great  effects  upon  the 
universe.  Great  effects  do  not  arise  if  that  which  is 
done  during  one  cycle  of  years  is  all  undone  during  the 
next.  The  tides  are,  however,  ever  in  operation  and  their 
influences  tend  continually  in  the  same  direction.  Con- 
sequently the  alteration  in  the  length  of  the  day  is 
continually  in  progress,  and  in  the  course  of  illimitable 
ages  its  effects  accumulate  to  a  startling  magnitude. 

§  84.  The  Earth's  Rotation  becomes  Slower.  The  Earth 
now  revolves  on  its  axis  once  in  twenty-four  hours.  There 
was  a  time,  most  likely  it  was  millions  of  years  ago,  when 
the  Earth  revolved  once  in  twenty-three  hours.  Earlier 


The  Moon  79 

still  it  must  have  spun  upon  its  axis  in  twenty -two  hours. 
The  very  same  arguments  applied  in  those  times  which 
apply  at  the  present,  so  that  as  we  look  back  further  and 
further  into  the  excessively  remote  past  we  find  the  Earth 
spinning  ever  more  and  more  rapidly,  until  at  last  we 
discern  an  epoch  when  the  length  of  the  day  having 
declined  to  eight  hours  and  seven  hours,  had  at  last  sunk 
to  something  like  five  or  six  hours.  This  is  the  time  at 
which  it  would  seem  that  the  history  of  the  Moon  may  be 
said  to  have  commenced,  when  the  Earth  was  accomplish- 
ing about  four  revolutions  in  the  same  time  that  it  now 
requires  for  a  single  revolution.  We  cannot  attempt  to 
assign  the  antiquity  of  this  critical  moment.  It  was  in  all 
probability  far  earlier  than  the  time  which  the  researches 
of  geologists  have  opened  out  to  us.  If  it  be  thought  that 
the  vagueness  of  our  knowledge  as  to  the  length  of  time 
which  these  changes  required  is  rather  unsatisfactory  then 
it  must  be  remembered  that  even  now  historians  who  have 
human  records  and  monuments  to  guide  them,  are  still 
often  in  utter  uncertainty  as  to  the  periods  during  which 
mighty  empires  flourished,  or  as  to  the  dates  at  which 
great  dynasties  rose  or  perished. 

§  85.  Reaction  on  the  Moon.  Among  the  profoundest 
laws  of  nature  is  that  which  asserts  that  action  and 
reaction  are  equal  and  opposite.  We  have  seen  that  the 
Moon  is  the  cause  of  the  tides.  And  we  have  further 
seen  how  the  tides  act  as  a  break  to  check  the  speed  with 
which  the  Earth  is  rotating.  This  is  the  action  of  the 
Moon  upon  the  Earth,  and  let  us  now  consider  the  nature 
of  the  reaction  with  which,  in  accordance  with  the  laws 
of  Mechanics,  this  action  must  be  inevitably  accompanied. 
The  Moon,  by  its  action  on  the  Earth,  through  the  medium 
of  the  tides,  tends  to  check  the  speed  with  which  the 
Earth  is  rotating  on  its  axis,  and  so  the  Earth  reacts  on 
t  lie  Moon  and  compels  its  satellite  to  adopt  a  continuous 


80  Astronomy 

retreat.  In  accordance  with  this  action  the  Moon  is  there- 
fore gradually  receding  from  the  Earth.  It  is  further  from 
the  Earth  to-day  than  it  was  yesterday,  it  will  be  further 
to-morrow  than  it  is  to-day.  The  process  is  never  reversed; 
it  never  even  ceases.  The  consequence  is  a  continuous 
growth  in  the  size  of  the  track  which  the  Moon  has  been 
describing  around  the  Earth  from  the  earliest  period 
of  its  history  up  to  the  present  day.  It  is  quite  true 
that  this  growth  in  the  diameter  of  the  Moon's  orbit 
has  been  but  slow,  yet  is  not  also  the  growth  of  the 
oak  tree  imperceptible  from  day  to  day,  though  in  the 
lapse  of  centuries  the  tree  attains  a  magnificent  stature  ? 
The  enlargement  of  the  Moon's  orbit,  though  impercep- 
tible from  month  to  month  or  even  from  century  to  cen- 
tury, has  revolutionised  our  system  in  the  lapse  of  many 
millions  of  years. 

§  86.  The  Moon  Retreats.  Looking  back  through 
these  immense  periods  of  time  we  see  the  Moon  ever 
drawing  nearer  and  nearer  to  the  Earth.  Our  satellite  now 
revolves  at  a  distance  of  two  hundred  and  forty  thousand 
miles,  but  there  was  a  time  when  that  distance  was  not 
greater  than  two  hundred  thousand  miles. .  There  was  a 
time  millions  of  years  ago,  no  doubt,  when  the  Moon  was 
but  a  hundred  thousand  miles  away,  and  as  we  look  back 
ever  further  and  further  to  the  early  stages  of  the  Earth- 
Moon  system  we  see  the  Moon  ever  drawing  closer  and 
closer  to  our  globe  until  at  last  we  discern  that  most 
critical  stage  in  Earth-Moon  history,  when  our  globe  was 
spinning  round  in  a  period  of  five  or  six  hours.  Instead  of 
revolving  in  a  distant  orbit,  which  has  been  the  case  ever 
since,  the  Moon  was  then  close  to  the  Earth,  was,  in  fact, 
actually  touching  our  globe,  and  the  two  bodies,  or  per- 
haps it  would  be  more  correct  to  say,  the  materials  of 
which  these  two  bodies,  as  we  now  know  them,  were  then 
formed,  were  revolving  in  contact,  each  around  the  other. 


The  Moo,,  81 

§  87.  Origin  of  the  Moon.  It  is  impossible  at  this 
point  to  resist  taking  one  step  further,  though  in  taking  it 
we  must  to  some  extent  dispense  with  the  safe  guidance  of 
mathematical  analysis  which  has  led  us  up  to  this  point. 
At  that  extremely  early  period  the  Earth  was  not  then  the 
solid  mass  which  we  now  know  so  well.  In  those  early 
ages  it  was  highly  heated,  it  was  heated  to  such  a  tem- 
perature that  instead  of  being  a  rigid  body  it  was  a  soft 
molten  mass  of  matter,  and  that  molten  globe  was  spinning 
round  four  times  as  fast  as  our  Earth  spins  at  present. 
The  speed  in  that  primitive  globe  seems  to  have  been  so 
great  that  a  rupture  took  place.  According  to  this  view 
a  part  of  the  molten  matter,  comparatively  small,  broke 
away  from  the  parent  globe  and  formed  into  a  small  globe 
adjoining  the  greater.  Thus  it  would  seem  that  the  Moon 
had  its  origin  in  a  disruption  of  the  Earth.  Such  is  the 
lesson  which  we  are  taught  by  the  movements  of  the  tides. 


CHAPTER   V 
GRAVITATION 

§  88.  Kepler's  Laws.  The  movements  of  the  planets 
around  the  Sun.  are  conducted  in  conformity  with  the 
three  fundamental  principles  which  are  known  as  the  laws 
of  Kepler.  As  a  first  approximation  it  may  be  said  that 
each  planet  moves  in  a  circular  orbit  around  the  Sun,  that 
each  planet  moves  round  its  circle  with  a  uniform  velocity, 
that  all  these  circles  are  in  the  same  plane,  and  that  each 
planet  revolves  in  the  same  direction  along  its  circle. 
When,  however,  more  careful  investigation  is  made  it  is 
found  that  the  tracks  of  the  planets  are  not  absolutely 
circular.  It  is  also  found  that  the  movements  of  the 
planets  along  their  different  orbits  are  not  absolutely 
uniform.  The  researches  of  Kepler  succeeded  in  shewing 
what  the  actual  shape  of  a  planet's  track  must  necessarily 
be  and  also  explained  the  law  according  to  which  the  speed 
of  the  planet  at  each  point  of  its  path  varies.  The  results 
of  these  famous  researches  are  embodied  in  those  propo- 
sitions which  from  the  name  of  their  author  are  known 
as  Kepler's  Laws. 

§  89.  First  Law.  The  first  of  the  three  discoveries 
which  lie  at  the  basis  of  modern  astronomy  may  be  thus 
enunciated. 


Gravitation  83 

The  path  of  a  planet  round  the  Sun  is  an  Ellipse,  in 
one  focus  of  which  the.  centre  of  the  Sun  is  situated. 

We  may  illustrate  this  by  the  adjoining  figure.  Let  S 
be  the  Sun  and  let  ABPQ  be 
the  ellipse.  The  focus  of  the 
ellipse  is  well  known  to  geo- 
meters ;  it  is  well  known  to 
the  draughtsman  also,  inas- 
much as  when  he  proceeds  to 
draw  an  ellipse  by  the  ordi- 
nary artifice  for  producing  the 
figure  which  we  have  already 
explained  in  §  53.  In  Fig.  18 
the  point  S  represents  one  of  Fig>  18<  The  Ellipse  and 
the  two  Foci.  It  is  not  neces-  Kepler's  Laws, 

sary  to  introduce  the  second 

focus,  for  only  one  of  the  foci  of  the  ellipse  is  involved 
in  the  enunciation  of  Kepler's  laws  of  planetary  motion. 
I  ought  indeed  to  say,  that  in  the  case  of  110  one  of  the 
important  planets  does  the  actual  track  depart  nearly  so 
far  from  the  circular  form  as  does  the  ellipse  which  we 
have  here  represented.  Such  then  is  the  first  of  Kepler's 
laws.  But  the  planet  as  it  moves  round  its  ellipse  changes 
its  speed  at  different  parts.  When  furthest  from  the  focus 
which  is  occupied  by  the  Sun  the  speed  of  the  planet  is  at 
its  lowest,  while  the  velocity  attains  its  maximum  when 
the  planet  is  passing  round  that  end  of  the  ellipse  near 
which  the  Sun.  is  situated. 

§  90.  Second  Law.  The  law  which  controls  these 
movements  was  also  discovered  by  Kepler,  and  has  teen 
enunciated  in  the  second  law,  which  may  IK?  thus  stated. 

The  straight  line  drawn  from  the  centre  of  the  Sun  to 
the  centre  of  the  planet  moves  over  equal  areas  in  equal 
times. 

To  illustrate  this  law  with  the  present  figure  I  take 


84  Astronomy 

two  points,  A,  B  on  the  track  of  the  planet  at  the  part 
most  distant  from  the  Sun,  and  two  other  points,  P,  Q  on 
that  part  of  the  track  which  lies  nearest  to  the  Sun.  The 
planet,  as  I  have  said,  is  moving  more  rapidly  in  the  latter 
case  than  in  the  former,  so  that  if  we  represent  by  PQ  the 
distance  through  which  the  planet  has  moved  in  a  given 
time  at  one  station,  and  if  we  take  AB  to  represent  the 
distance  through  which  the  planet  has  moved  in  a  given 
time  at  the  other  end  of  the  orbit,  Kepler's  second  law 
asserts  that  when  these  times  are  equal  the  area  inter- 
cepted between  the  two  straight  lines  SP,  SQ  and  the 
ellipse  is  equal  to  the  area  intercepted  between  the  two 
straight  lines  SA,  SP  and  the  ellipse.  The  same  law 
governs  the  changes  in  the  planet's  speed  at  all  parts  of 
its  orbit.  So  that  if  the  area  >SXFis  equal  to  SAB  then 
the  planet  will  take  the  same  time  to  pass  from  Xto  Fas 
from  A  to  B. 

§  91.  Third  Law.  The  third  law  of  Kepler  differs 
from  the  two  preceding  ones  inasmuch  as  now  we  compare 
together  the  movements  of  the  different  planets.  Kepler's 
third  law  is  to  give  precision  to  the  fact  that  had  been 
early  noticed,  namely  that  the  further  a  planet  was  from 
the  Sun  the  longer  the  time  it  took  to  accomplish  a  single 
revolution.  This  would  of  course  be  naturally  expected 
from  the  circumstance  that  the  larger  the  orbit  the  longer 
the  journey,  and  therefore  if  each  of  two  planets  was 
animated  with  the  same  velocity  the  time  required  for  the 
greater  journey  would  be  longer  than  the  time  required  for 
the  less.  As  a  matter  of  fact,  however,  the  more  distant 
planet  from  the  Sun  does  not  move  so  rapidly  as  does  the 
nearer  planet.  It  therefore  follows  for  a  double  reason 
that  the  periodic  time  in  the  greater  orbit  would  be  longer 
than  the  periodic  time  of  the  planet  in  the  smaller  orbit. 
Kepler's  third  law  has  given  precision  to  these  inferences. 
His  law  is  enunciated  in  this  way. 


Gravitation  85 

TJie  squares  of  the  periodic  times  of  two  planets  are  in 
the  same  ratio  as  the  cubes  of  their  mean  distances  from  the 
Sun. 

It  should  be  explained  that  in  the  expression  of  this 
law  we  are  to  understand  by  the  mean  distance  of  the 
planet  from  the  Sun  a  length  equal  to  the  semi-axis-major 
of  the  ellipse,  which  in  accordance  with  the  first  law  is 
the  actual  shape  of  the  orbit.  The  periodic  time  may  be 
expressed  in  the  number  of  days  and  fractions  of  a  day 
which  the  planet  requires  to  effect  a  complete  revolution 
around  the  Sun. 

§  92.  Illustration  of  Kepler's  third  Law.  The  con- 
venience of  Kepler's  third  law  arises  from  the  fact  that 
when  the  periodic  times  of  the  planets  are  known,  we  can 
deduce  the  relative  values  of  their  mean  distances.  We 
may  illustrate  the  application  of  the  law  by  taking  the 
case  of  Venus  and  the  Earth.  The  periodic  time  of  Venus 
is  224-7  days,  and  of  the  Earth  365-3  days.  If  we  divide 
the  latter  quantity  into  the  former  and  take  the  square 
of  the  quotient  we  obtain  the  figure  0-37835.  This  figure 
therefore,  according  to  Kepler's  third  law,  must  represent 
the  ratio  which  the  cube  of  the  mean  distance  of  Venus 
bears  to  the  cube  of  the  mean  distance  of  the  Earth.  If 
we  represent  the  latter  by  unity  then  this  number  is  the 
cube  of  the  mean  distance  of  Venus  expressed  in  the  same 
units.  But  0-37835  is  the  cube  of  0-7233.  Hence  we  see 
that  the  relative  distances  of  the  Earth  and  Venus  are  as 
1  to  0-7233. 

§  93.  Newton's  Law  of  Gravitation.  Kepler  discovered 
his  wonderful  laws  by  the  most  diligent  comparison  of 
such  observations  of  the  planets  as  were  available  to  him. 
These  laws  are  monuments  of  ingenious  and  painstaking 
labour.  But  Kepler  had  no  grounds  for  seeing  why  these 
laws  rather  than  other  laws  should  really  be  the  guiding 
principles  of  the  planetary  movements.  It  was  reserved 


for  Newton  to  lay  down  the  fundamental  law  of  Gravita- 
tion, by  which  these  laws  were  demonstrated  as  conse- 
quences of  the  principle  that  every  particle  in  the  universe 
attracts  every  other  particle  ivith  a  force  whose  intensity  varies 
directly  as  the  product  of  the  masses  of  the  two  particles  and 
inversely  as  the  square  of  their  distance  from  one  another. 

§  94.  First  Law  of  Motion.  The  first  law  of  motion, 
as  laid  down  by  Newton,  affirms  that  a  moving  body,  not 
acted  upon  by  any  force,  would  for  ever  continue  to  move 
along  uniformly  in  a  straight  line.  If  then  a  body  were 
found  to  be  moving  in  a  curved  line,  or  even  while  mov- 
ing in  a  straight  line  if  the  velocity  of  the  body  was  not 
uniform,  then  it  was  certain  that  some  force  must  be  in 
operation  on  that  body.  It  was  obvious  that  the  planets 
move  neither  in  straight  lines  nor  with  uniform  velocities, 
and  therefore  it  was  certain  that  the  planet  must  be  acted 
upon  by  certain  forces.  When  Kepler's  laws  had  exhibited 
in  the  most  precise  manner  what  the  actual  nature  of  the 
curve  which  the  planets  pursued  was,  and  when  he  had 
further  shewn  the  law  according  to  which  the  velocity  of 
each  planet  varied  at  the  different  points  of  its  track  he 
had  laid  the  foundation  on  which  Newton  subsequently 
developed  his  grand  theory.  Kepler  having  demonstrated 
that  the  planet  describes  equal  areas  around  the  Sun  in 
equal  times,  Newton  was  able  to  shew  that  from  this 
circumstance  alone  it  was  actually  demonstrable  that  the 
planet  must  be  acted  upon  by  a  force,  and  that  the  force 
must  be  always  directed  from  the  Sun.  Here  was  indeed 
a  remarkable  advance.  It  was  shewn  that  the  character- 
istic feature  which  regulated  the  variation  in  the  planet's 
velocity  could  be  accounted  for,  and  could  only  be  ac- 
counted for  by  the  supposition  that  the  force  by  which 
the  planet  was  controlled  emanated  directly  from  the  Sun. 
The  next  question  was  as  to  the  way  in  which  the  force 
from  the  Sun  varied.  Here  mathematical  calculations 


87 

were  possible,  We  can  assume  a  certain  law  of  force  and 
we  can  shew,  according  to  each  law  of  force,  what  particular 
track  the  planet  would  pursue.  Knowing  that  the  track 
is  an  ellipse  and  that  the  Sun  occupies  the  focus  of  that 
ellipse,  Newton  then  demonstrated  that  the  attraction  of 
the  Sun  on  the  planet  must  vary  inversely  as  the  square 
of  the  distance.  If  the  force  varied  according  to  any  other 
law,  then  the  orbit  would  not  be  an  ellipse,  or  if  an  ellipse 
the  Sun  would  not  in  such  case  lie  at  the  focus.  It  thus 
followed  from  Kepler's  first  and  second  law  that  each 
planet  was  attracted  by  the  Sun  and  that  the  force  with 
which  the  Sun  attracts  the  planet  varies  inversely  as  the 
square  of  the  distance.  This  was  the  greatest  of  all  New- 
ton's achievements,  described  in  his  immortal  Principia,. 
in  explanation  of  the  phenomena  of  nature.  The  discov- 
ery of  the  law  of  universal  attraction  lies  at  the  basis  of 
all  mathematical  astronomy. 

§  95.  Second  Law  of  Motion.  Just  as  the  planets  re- 
volve around  the  Sun  in  obedience  to  this  law,  so  the  Moon 
describes  an  orbit  around  the  Earth.  This  orbit  is  nearly 
circular  but  more  accurate  measurement  shews  that  it 
must  be  regarded  as  an  ellipse.  In  like  manner  the  satel- 
lites of  the  other  planets  describe  ellipses  around  their 
primaries.  The  memorable  investigation  by  which  Newton 
shewed  that  the  gravitation  of  the  Moon  towards  the  Earth 
was  a  force  of  the  same  nature  as  the  gravitation  by  which 
the  various  planets  are  guided  round  the  Sun,  must  now  be 
explained.  The  first  step  is  to  shew  that  the  force  which 
retains  the  Moon  in  its  orbit  is  really  that  same  attraction 
of  the  Earth  which  causes  a  body  to  fall  when  released  near 
its  surface.  Newton's  second  great  law  of  motion  is  that 
the  motion  communicated  to  a  body  by  a  force  acting  upon 
it  is  proportional  to  the  intensity  of  the  force,  and  from  this 
it  follows  that  the  distance  through  which  a  body  will 
move  in  the  first  second  under  the  action  of  a  force  is  a 


88  Aatronomg 

measure  of  the  force.  It  is  known  by  experiment  that  a 
body  let  fall  near  the  surface  of  the  Earth  will  drop  through 
sixteen  feet  in  the  first  second.  The  distance  of  the  Moon 
from  the  Earth  is  about  sixty  radii  of  the  Earth,  and  as 
the  law  of  Gravitation  declares  that  the  intensity  of  the 
attraction  varies  inversely  as  the  square  of  the  distance  it 
is  easy  to  calculate  that  a  body  let  fall  towards  the  Earth 
at  the  distance  of  the  Moon  would  in  each  second  fall 
towards  the  Earth  through  a  distance  which  is  found  by 
dividing  sixteen  feet  by  3600.  It  thus  appears  that  at 
this  distance  a  body  will  fall  under  gravity  through  about 
one-twentieth  of  an  inch  in  the  first  second.  If  the  action 
of  the  Earth's  gravitation  upon  the  Moon  were  to  cease  at 
any  moment,  then  in  conformity  with  the  first  law  of 
motion  the  Moon  would  move  on  for  ever  in  a  straight  line 
which  would  be  the  tangent  to  its  orbit  at  the  spot  where 
the  Moon  was  situated  at  the  moment  when  the  action 
was  ari-ested.  But  in  consequence  of  the  attraction  of 
the  Earth,  the  Moon  is  compelled  to  swerve  from  the 
straight  line  and  to  adopt  instead  the  nearly  circular  orbit 
in  which  we  know  that  it  moves.  It  is  easy  to  calculate 
the  amount  by  which  the  centre  of  the  Moon  is  drawn  in 
this  way  towards  the  Earth  in  the  course  of  a  second. 
It  is  found  that  the  Moon  is  actually  at  the  end  of  each 
second  about  one-twentieth  of  an  inch  nearer  to  the  centre 
of  the  Earth  than  it  would  have  been  if  the  Earth's 
attraction  had  not  helped  it.  This  is  precisely  what  the 
law  of  Gravitation  would  require  and  consequently  we  are 
assured  that  the  force  which  makes  a  stone  fall  when 
released  near  the  Earth's  surface  is  precisely  the  same 
force,  suitably  reduced,  however,  in  accordance  with 
Newton's  Law,  which  controls  the  monthly  movement 
of  the  Moon. 

§  96.    Planetary  Perturbations.     One  of  the  most  in- 
teresting applications  of  the  principle  of  gravitation  is  to 


Gravitation  89 

the  explanation  of  what  are  known  as  the  perturbations 
of  the  movements  of  the  planets.  The  Sun  is  so  great 
that  it  enormously  exceeds  in  mass  all  the  planets  taken 
together.  We  may,  therefore,  on  a  first  view,  regard  the 
influence  of  the  Sun  as  so  predominant  in  the  Solar  sys- 
tem that  the  planets  move  entirely  in  accordance  with  its 
guidance.  It  must  however  be  remembered  that  the  law 
of  gravitation  affirms  that  every  particle  in  the  Universe 
attracts  every  other  particle.  And  consequently  the  planet 
Jupiter,  for  instance,  is  not  alone  guided  in  its  elliptic 
path  by  the  attraction  of  the  Sun,  but  is  affected  to  a 
certain  extent  by  the  attractions  of  the  other  planets. 
The  Earth  and  Venus,  Mars  and  Saturn,  Uranus  and 
Xeptune,  each  exercise  distinct  effects  upon  the  move- 
ments of  Jupiter.  The  consequence  of  this  is  that 
although  the  planet  has  a  motion  which  is  in  the  main 
that  which  would  be  produced  by  the  attraction  of  the 
Sun  only,  yet  when  its  movements  are  closely  looked 
into  there  are  seen  to  be  slight  divergences  between  the 
simple  elliptic  movement  that  would  be  conducted  in 
accordance  with  Kepler's  laws  and  the  actual  movements 
which  the  telescope  shews  to  belong  to  the  planet.  The 
theory  of  universal  gravitation  has  generally  reconciled 
the  theory  and  the  observations.  Discrepancies  in  the 
calculated  places  of  the  planet,  as  those  places  would  be 
if  the  motion  were  purely  elliptic,  are  usually  seen  to 
agree  with  the  calculated  amounts  of  the  disturbances 
which  the  other  planets  would  produce. 

•  The  most  illustrious  mathematicians  have  expended 
their  powers  on  the  study  of  this  great  subject,  and  many 
magnificent  discoveries  have  rewarded  their  labours.  It 
has  been  shewn  for  example  that  although  a  planet  does 
not  move  exactly  in  an  ellipse,  yet  when  long  periods  of 
time  are  concerned  the  motion  of  the  planet  may  be  most 
perfectly  represented  by  movements  in  an  ellipse,  if  we 


00  Artmnamy 

make  the  additional  supposition  that  the  ellipse  itself  is 
not  constant  in  form  or  in  position,  but  that  it  is  under- 
going slight  changes.  According  to  this  view  we  always 
think  of  a  planetary  movement  as  taking  place  in  an 
ellipse,  but  in  an  ellipse  which  is  itself  undergoing  slow 
changes.  Some  propositions  have  been  arrived  at  by 
mathematical  research  which  exhibit  the  nature  of  these 
movements  in  the  very  simplest  manner.  I  shall  sup- 
pose for  instance  that  we  are  now  considering  only  two 
planets,  whose  mutual  disturbance  affects  the  simplicity 
that  the  orbits  would  otherwise  possess.  It  was  shewn 
by  the  great  mathematician,  Lagrange,  that  notwithstand- 
ing the  incessant  attraction  of  one  planet  on  the  other, 
and  notwithstanding  the  many  other  changes  which  the 
ellipse  would  undergo,  yet  there  was  one  very  important 
feature  in  the  ellipse  which  would  undergo  no  change. 
It  was  shewn  that  the  length  of  the  axis  of  the  ellipse 
Avould  be  constant.  It  was  also  proved,  in  the  case  of 
two  large  planets  that  though  they  might  each  affect  the 
eccentricity  of  the  other's  orbit  yet  that  the  fluctuations 
of  those  eccentricities  would  be  always  restricted  within 
narrow  limits.  It  was  shewn  that  if  the  orbits  were  once 
nearly  circular  in  their  form  they  would  always  remain 
nearly  circular  in  their  form.  It  was  further  demonstrated 
that  though  the  inclinations  of  the  orbits  to  each  other 
might  undergo  slight  changes,  yet  that  those  changes  could 
only  be  slight.  It  should  be  added  that  these  latter  re- 
sults are  only  true  on  the  supposition  that  the  planets 
revolve  around  the  Sun  in  the  same  direction.  If  the 
directions  in  which  the  two  planets  revolved  were  differ- 
ent, that  is  to  say,  if  one  of  the  planets  was  revolving  in 
the  same  direction  as  the  hands  of  a  clock,  while  the  other 
planet  was  revolving  in  the  opposite  direction,  then  these 
guarantees  for  the  stability  of  the  system  would  not  be 
given.  As,  however,  we  find  the  planets  all  do  revolve  in 


Gravitation  91 

the  same  direction  we  are  assured  that  in  the  case  of  the 
principal  planets  at  least  the  magnitudes  of  their  orbits  as 
measured  by  the  dimensions  of  the  principal  axes,  shall 
remain  unchanged,  while  the  eccentricities  of  those  orbits, 
as  well  as  their  mutual  inclinations,  will  only  vary  between 
very  narrow  limits.  It  need  hardly  be  said  that  these 
propositions  are  of  the  utmost  importance  to  a  planet 
considered  as  an  abode  of  organised  life.  It  is  obviously 
essential  for  the  welfare  of  the  inhabitants  of  the  Earth 
that  the  Earth  should  remain  in  or  about  the  same  distance 
from  the  Sun  and  that  the  succession  of  its  seasons  should 
not  be  subject  to  such  extreme  variations  as  might  arise 
if  the  eccentricity  of  the  orbit,  or  its  inclination,  were 
seriously  altered.  The  assurance  that  no  important  altera- 
tions can  arise  has  been  given  by  the  labours  of  the  great 
mathematicians,  especially  Lagrange  and  Laplace. 

§  97.  Determination  of  the  Mass  of  a  Planet.  We 
are  also  indebted  to  the  principle  of  gravitation  for  our 
means  of  solving  the  very  important  problem  of  finding 
the  masses  of  the  different  bodies  in  the  solar  system.  In 
the  first  place  it  is  desirable  to  compare  the  mass  of  the 
Earth  with  the  mass  of  the  Sun.  We  have  already  ex- 
plained how  the  attraction  of  the  Earth  is  exhibited  in 
making  a  body  at  its  surface  fall  16  feet  in  the  first 
second.  We  know,  however,  that  the  average  distance 
of  the  Sxm  is  nearly  equal  to  23300  radii  of  the  Earth. 
If  therefore  the  attraction  of  the  Earth  acted  on  a  body 
revolving  around  it  at  this  distance  it  would  make  that 
body  fall  towards  it  in  the  course  of  a  single  second 
through  a  distance  which  was  equal  to  16  feet  divided 
by  the  square  of  23300.  But  just  as  we  were  able  to  find 
the  distance  through  which  the  Moon  falls  in  towards  the 
Earth  by  considering  the  size  of  the  Moon's  orbit  and  the 
length  of  the  month  (§  9o),  so  we  are  able  to  calculate 
the  distance  through  which  the  Earth  falls  in  one  second 


92  Astronomy 

towards  the  Sun.  We  find  in  this  way  that  the  distance 
through  which  the  Earth  falls  towards  the  Sun  in  the 
course  of  a  single  second  is  324,000  times  as  great  as  the 
distance  through  which  the  Earth  would  cause  a  body 
revolving  at  the  same  distance  to  fall.  Therefore  the 
attraction  of  the  Sun  must  be  324,000  times  as  great  as 
that  of  the  Earth.  And  hence,  taking  the  mass  of  the 
Sun  as  unity,  we  have  for  the  mass  of  the  Earth  the 
fraction  ^TOTO- 

§  98.  Mass  of  a  Planet  with  a  Satellite.  In  the  process 
of  weighing  a  planet  we  must  make  a  distinction  between 
those  planets  which  are  provided  with  attendant  satellites 
and  those  which  are  not  so  accompanied.  In  the  former 
case  the  problem  of  determining  the  mass  of  the  planet 
offers  but  little  difficulty.  The  movements  of  the  satellites 
have  been  carefully  examined  by  astronomers  and  thus  the 
distance  through  which  the  satellite  falls  towards  the 
planet  in  each  second  is  easily  computed.  We  also  know 
from  the  movement  of  the  planet  itself  the  distance 
through  which  it  drops  towards  the  Sun  in  the  course 
of  a  second.  And  hence  we  have  the  simple  method  of 
finding  the  ratio  of  the  mass  of  the  planet  to  the  mass 
of  the  Sun. 

§  99.  Mass  of  a  Planet  without  a  Satellite.  Much  more 
difficult  is  the  problem  of  ascertaining  the  mass  of  a 
planet  which,  like  Venus  or  like  Mercury,  is  not  attended 
by  known  satellites.  The  only  method  of  solving  the  prob- 
lem in  this  case  is  to  determine  the  perturbations  in  the 
movements  of  the  other  planets  which  are  produced  by 
the  action  of  these  bodies.  The  amount  of  perturbation 
that  the  Earth,  let  us  say,  experiences  by  the  action  of 
Venus  depends  on  the  mass  of  Venus,  and  if  therefore 
observations  are  made  as  to  the  extent  to  which  the  Earth 
has  been  disturbed  by  this  planet,  then  we  have  an  in- 
dication of  what  the  mass  of  the  planet  must  be,  by  whose 


Gravitation  93 

attraction  that  particular  disturbance  has  been  caused. 
This  method  seems  very  elaborate,  110  doubt,  but  yet  it 
admits  of  considerable  accuracy.  A  striking  verification 
of  its  utility  was  provided  in  the  case  of  the  planet  Mars. 
Until  the  discovery  of  the  satellites  of  this  planet  in  1877 
our  only  knowledge  of  the  mass  of  Mars  was  inferred  from 
the  perturbations  which  that  planet  caused  in  the  move- 
ments of  its  neighbours.  .  When  these  interesting  satel- 
lites were  discovered  the  mass  of  the  planet  was  at  once 
obtained  by  the  simpler  and  more  accurate  method.  It 
was,  however,  foiind  that  the  value  of  the  mass  which  was 
obtained  from  the  observations  of  the  satellites  was  in 
practical  agreement  with  the  mass  which  had  been  de- 
duced from  the  perturbations. 

§  100.  Superficial  Gravity.  The  intensity  of  gravita- 
tion at  the  surface  of  a  celestial  body  will  depend  of  course 
on  the  mass  of  that  body.  But  it  will  also  depend  on  the 
body's  density,  for  the  less  the  density  the  greater  the 
bulk  of  the  body  corresponding  to  a  given  mass,  and 
consequently  the  greater  is  the  distance  of  any  particle 
on  its  surface  from  its  centre.  It  is  interesting  to  note 
the  variations  in  the  gravitation  at  the  surfaces  of  the 
different  bodies  of  the  solar  system.  Thus,  for  instance, 
in  the  case  of  the  Sun,  it  can  be  shewn  that  the  gravitation 
at  the  surface  of  that  great  globe,  is  twenty-seven  times  as 
great  as  the  gravitation  at  the  surface  of  the  Earth.  We 
mean  by  this  that  if  we  have  a  mass  which  weighs  one 
pound  upon  the  Earth  then  to  sustain  its  weight  at  the 
surface  of  the  Sun  would  call  for  as  much  effort  as  would 
suffice  to  sustain  a  weight  of  twenty-seven  pounds  on  the 
surface  of  the  Earth.  We  may  put  the  matter  in  another 
way.  Suppose  that  a  weight  was  suspended  from  a  spring 
balance,  and  that  the  same  weight  and  the  same  spring 
balance  were  transferred  to  the  surface  of  the  Sun.  It 
would  then  appear  that  the  indication  of  the  spring 


94  Astronomy 

balance,  as  given  on  the  Earth,  would  only  amount  to 
one-twenty-seventh  part  of  what  the  same  balance  would 
indicate  on  the  Sun. 

The  mass  of  Jupiter  is  three  hundred  times  as  great 
as  the  mass  of  the  Earth,  but  owing  to  the  vast  bulk  of 
Jupiter  and  the  low  specific  gravity  of  his  materials  the 
intensity  of  gravitation  at  the  surface  of  the  planet  is  not 
nearly  so  great  as  might  be  expected  from  his  great  mass ; 
it  is  indeed  no  more  than  about  two  and  a  half  times  as 
great  as  the  gravitation  on  the  surface  of  the  Earth. 

An  instance  of  the  opposite  kind  is  presented  by  the 
Moon.  The  mass  of  the  Moon  being  only  about  one- 
eightieth  part  of  the  mass  of  the  Earth,  it  would  be 
expected  of  course  that  a  body  should  weigh  less  at  the 
surface  of  the  Moon  than  the  same  body  would  weigh  here. 
On  the  other  hand  the  diameter  of  the  Moon  is  only  about 
a  fourth  of  the  diameter  of  the  Earth.  The  consequence 
is,  reserving  only  round  numbers,  that  the  weight  of  a 
body  on  the  Moon  is  only  a  sixth  of  the  weight  Avhich  the 
same  body  has  here.  A  labourer  could,  for  instance,  carry 
six  times  the  load  on  the  Moon  that  he  could  carry  with 
the  same  exertion  on  the  Earth. 

§  101.  Perturbations  of  the  Moon's  Orbit.  Some  very 
interesting  illustrations  of  universal  gravitation  are  af- 
forded by  the  movements  of  the  Moon.  Just  as  the  planets 
are  disturbed  in  their  movements  around  the  Sun  by  the 
attractions  of  the  other  planets,  so  the  movements  of  the 
Moon  around  the  Earth  are  also  incessantly  undergoing 
perturbations.  These  perturbations  of  the  Moon  have 
however  been  produced  very  differently  from  the  pertur- 
bations of  the  planets.  We  have  already  explained  how 
the  planets  attract  one  another.  No  doubt  the  planets 
also  attract  the  Moon,  but  their  effect  in  disturbing  the 
movements  of  our  satellite  may  be  considered  as  insensible. 
The  disturbance  of  the  Moon's  orbit  around  the  Earth 


Gravitation  95 

arises  from  the  attraction  of  a  much  more  important  body. 
It  is  the  attraction  of  the  Sun  itself.  The  Moon  is  so  close 
to  the  Earth  that  the  attraction  of  the  Earth  is  the  domi- 
nating influence,  and  the  movements  of  the  Moon  are 
mainly  controlled  by  the  Earth.  The  Sun  is  about  four 
hundred  times  as  far  from  us  as  the  Moon,  and  conse- 
quently the  attractive  force  of  the  Sun  or  rather  its  dis- 
turbing effect  is  greatly  lessened ;  still,  owing  to  the  large 
mass  of  the  Sun  its  disturbance  of  the  Moon  is  quite  con- 
siderable, even  at  that  distance.  Many  irregularities  in 
the  Moon's  movements  have  to  be  attributed  to  the  dis- 
turbing solar  effect.  I  may  mention  one  of  them  which 
was  discovered  by  Tycho  Brahe. 

§  102.  Annual  Equation.  Let  us  just  think  of  the 
case  of  Full  Moon.  In  that  phase  the  Moon  is  at  the 
further  side  of  the  Earth  from  the  Sun,  and  the  attrac- 
tion of  the  Sun  on  the  Earth  is  therefore  greater  than  its 
attraction  on  the  more  distant  Moon.  The  Earth  is  there- 
fore more  drawn  to  the  Sun  than  is  the  Moon  and  thus  the 
distance  between  the  Earth  and  the  Moon  is  increased. 
At  the  time  of  the  first  quarter,  however,  the  Earth  and 
the  Moon  are  both  at  practically  the  same  distance  from 
the  Sun.  They  are  consequently  both  drawn  by  the  same 
force  and  as  the  disturbing  force  acts  in  lines  which  are 
very  nearly  parallel  there  is  but  little  tendency  under  these 
circumstances  to  alter  the  distances  of  the  two  bodies.  On 
the  other  hand,  at  New  Moon  the  Moon  is  nearer  the  Sun 
than  the  Earth  is,  and  consequently  the  Moon  is  more 
drawn  towards  the  Sun  than  is  the  Earth,  and  therefore 
again,  the  tendency  of  the  solar  attraction  is  to  increase 
the  distance  between  these  two  bodies.  Thus  on  the  whole 
the  tendency  of  the  disturbing  solar  effect  is  to  increase  the 
size  of  the  Moon's  orbit.  For  we  have  seen  that  at  New 
Moon  and  Full  Moon  that  orbit  is  increased  while  at  the 
first  quarter  and  the  last  there  is  no  tendency  to  change  it. 


96  Astronomy 

As  however  the  Earth  is  revolving  around  the  Sun  in  an 
elliptic  path  the  Earth  is  sometimes  nearer  the  Sun  than 
on  other  occasions.  When  the  Earth  is  in  Perihelion,  being 
then  nearest  to  the  Sun,  the  disturbing  effect  on  the  Earth 
and  Moon  is  a  maximum.  On  the  other  hand,  when  the 
Earth  is  in  Aphelion,  then  our  globe  being  at  its  greatest 
distance  from  the  Sun,  this  disturbing  effect  is  a  minimum. 
Now  we  have  seen  how  the  disturbing  effect  of  the  Sun 
tends  to  increase  the  size  of  the  Moon's  orbit  and  thus 
tends  to  increase  the  Moon's  periodic  time.  We  have  also 
seen  how  the  action  of  the  Sun  in  producing  this  increase 
of  the  Moon's  periodic  time  is  of  greater  efficiency  at  the 
time  of  perihelion  than  at  the  time  of  aphelion.  We  have 
consequently  a  certain  annual  alteration.  And  this  pro- 
duces its  effect  in  a  change  of  the  apparent  place  of  the 
Moon  from  what  that  place  would  be  if  the  Moon  pursued 
its  course  without  experiencing  a  solar  disturbance,  or  if 
this  solar  disturbance  acted  equally  at  all  times  of  the  year. 
§  103.  Secular  Acceleration  of  the  Moon.  One  of 
the  most  famous  of  the  irregularities  in  the  Moon's 
motion  is  due  to  what  is  called  the  secular  acceleration. 
We  have  seen  how  the  annual  equation  arises  from  the 
eccentricity  of  the  Earth's  orbit.  As  however  the  planets 
are  continually  acting  upon  the  Earth  they  produce  changes 
in  the  eccentricity.  And  as  the  eccentricity  affects  the 
annual  variation,  it  follows  that  in  this  way  the  influence 
of  the  planets  is  propagated  into  the  annual  equation. 
It  can  also  be  shewn  that  the  greater  the  eccentricity  of 
the  orbit  of  the  Earth  the  larger  is  the  orbit  of  the  Moon, 
and  vice  versd.  Under  present  conditions  the  effect  of 
the  planetary  disturbances  is  to  cause  a  gradual  decline 
from  century  to  century  in  the  eccentricity  of  the  Earth's 
orbit.  We  have  accordingly  a  correspondingly  gradual 
decrease  in  the  size  of  the  Moon's  orbit.  And  with  the 
lessening  of  that  orbit  comes  a  decrease  in  the  periodic 


time  of  the  revolution  of  the  Moon.  This  phenomenon 
has  been  brought  to  light  by  the  comparison  of  the  observa- 
tions of  ancient  eclipses  with  the  calculated  movements  of 
the  Moon.  About  one-half  of  the  observed  acceleration  of 
the  Moon's  motion  can  be  accounted  for  by  these  planetary 
perturbations. 

§  104.  Precession  of  the  Equinoxes.  We  are  also 
indebted  to  the  theory  of  gravitation  for  the  explana- 
tion of  a  remarkable  celestial  phenomenon  which  had 
been  noticed  ages  before  the  cause  of  the  phenomenon  was 
understood.  If  the  Earth  were  a  perfect  sphere  then  the 
attraction  of  any  celestial  body,  such  as  the  Sun  or 
Moon,  would  act  through  the  centre  of  that  sphere,  and 
the  rotation  of  the  body  would  remain  unaffected.  But 
the.  Earth  is  not  a  perfect  sphere,  it  is  protuberant  at  the 
Equator,  and  the  consequence  is  that  the  attraction  of  the 
8 uii  and  Moon  on  that  protuberant  part  is  responsible  for 
certain  irregularities  in  the  position  of  the  Earth's  axis  of 
rotation  which  cause  what  is  known  as  the  precession  of 
the  equinoxes. 

The  phenomenon  of  precession  may  be  illustrated 
by  the  common  peg  top,  which  when  set  spinning  will 
rotate  about  the  axis  of  symmetry  through  the  top  itself, 
and  under  certain  circumstances  that  axis  will  describe 
a  cone  with  a  slow  motion.  The  axis  of  the  Earth  has 
a  somewhat  corresponding  movement.  While  the  Earth 
rotates  rapidly  around  its  axis  once  in  the  course  of  each 
sidereal  day,  the  axis  itself  describes  a  slow  conical  move- 
ment, in  consequence  of  which  the  celestial  pole  moves 
round  a  small  circle  on  the  sphere  in  a  period  of  25,800 
years.  The  effect  of  this  precessional  movement  of  the 
Pole  on  the  right  ascension  and  declination  of  stars  has 
to  be  taken  account  of  by  the  practical  astronomer.  We 
shall  describe  the  effect  of  precession  on  the  apparent 
places  of  the  stars  in  a  later  chapter. 
ii 


CHAPTER   VI 

MERCURY  AND  VENUS 

§  105.  Planets  near  the  Sun.  The  several  planets 
revolve  around  the  Sun  in  the  centre  in  orbits  which  in 
the  case  of  the  important  planets  are  nearly  circular.  The 
orbits  are  represented  in  the  accompanying  figure.  The 


Fig.  19.    The  Inner  Planets. 

planet  nearest  to  the  Sun  is  Mercury,  which  revolves  in  a 
period  of  88  days.  Then  comes  Venus  with  a  period  of 
225  days  ;  next  follows  the  Earth  ;  and  then  Mars  with  a 
period  of  687  days ;  these  constitute  the  so-called  inner 


Mercury  and  Venus  99 

planets.  They  are  succeeded  by  the  minor  planets  or 
asteroids,  after  which  as  will  be  shewn  later  we  come  to 
the  mighty  planets  of  the  system. 

The  innermost  planet  Mercury  is  a  comparatively  small 
object,  having  a  diameter  only  three-eighths  that  of  the 
Earth.  This  object  is  so  close  to  the  Sun  that  for  by  far 
the  greater  part  of  each  revolution  it  is  too  close  to  the 
Sun  to  be  visible.  Even  when  Mercury  is  seen  it  cannot 
be  studied  with  the  same  minuteness  as  other  members  of 
the  system  which  are  not  only  larger  but  also  much  better 
situated  for  telescopic  scrutiny.  Mercury  is  however 
frequently  to  be  seen  either  immediately  after  sunset  or 
directly  before  sunrise ;  the  almanac  will  give  the  proper 
times.  But  in  no  case  is  it  of  such  interest  as  the  other 
inner  planets  Venus  and  Mars,  to  the  consideration  of 
which  we  now  proceed. 

§  106.  Size  of  Venus.  The  Earth  and  the  planet 
Venus  so  closely  resemble  each  other  that  it  is  doubtful 
whether  we  know  in  the  whole  Universe  of  two  celestial 
bodies  so  nearly  identical.  To  begin  with,  the  twin  pair  of 
planets  are  very  nearly  of  the  same  size.  The  diameter  of 
the  Earth  is  7918  miles,  while  that  of  Venus  is  7660  miles. 
We  thus  see  that  our  globe  is  somewhat  greater  than  Venus, 
but  ho\v  slight  is  the  difference  in  the  two  diameters.  Jt 
amounts  to  no  more  than  the  thirtieth  part  of  the  whole. 
I  f  a  pair  of  billiard  balls  had  no  greater  proportional  differ- 
ence in  their  sizes  than  the  difference  between  Venus  and 
the  Earth  it  would  be  hardly  possible  to  detect  the  difference 
between  one  ball  and  the  other  without  measurement. 

§  107.  Weight  of  Venus.  As  to  the  relative  weights 
of  the  two  globes,  we  must  admit  that  here,  to  a  very  certain 
extent,  we  are  not  on  very  sure  ground.  It  is  not  an  easy 
matter  to  determine  accurately  the  weight  of  the  Earth,  it 
is  still  more  difficult  to  find  the  weight  of  Venus.  But  so 
far  as  our  knowledge  goes,  the  resemblance  which  the  two 


100  Astronomy 

planets  shew  in  bulk  extends  also  to  their  weight.  The 
Earth  appears  to  be  but  little  heavier  than  Venus,  in  fact. 
in  weight  no  less  than  in  size,  the  two  globes  may  be 
described  as  astonishingly  alike. 

§  108.  Sun's  Light  and  Heat  on  Venus.  As  to  the 
direct  supply  of  heat  and  light  from  the  Sun  the  sister 
planet  Venus  is  rather  more  highly  favoured  than  our  Earth. 
It  can  be  easily  calculated  that  the  fervour  and  brightness 
of  the  sunbeams  falling  on  Venus  are  about  double  as 
intense  as  those  which  are  received  by  this  Earth  from  the 
same  source.  There  is  however  another  consideration  which 
must  here  be  taken  into  account.  The  period  required  by 
Venus  to  complete  one  of  its  revolutions  around  the  Sun  is 
only  225  days.  It  therefore  follows  that  the  seasons  on  the 
sister  planet  run  through  their  changes  much  more  rapidly 
than  do  the  corresponding  seasons  on  the  Earth.  The 
summer,  for  instance,  would  not  be  two-thirds  the  length 
of  our  summer.  Perhaps,  however,  the  extra  warmth  and 
light  received  by  the  planet  would  serve  as  a  compensation, 
so  that  supposing  that  planet  was  the  home  of  vegetation, 
the  seed-time  and  the  harvest  could  succeed  each  other  on 
Venus  more  rapidly  than  is  the  case  on  our  globe  where  the 
sunbeams  are  showered  down  so  much  more  sparingly. 

§  109.  Rotation.  Considering  that  our  Earth  turns 
round  once  in  twenty-four  hours,  thus  producing,  of  course, 
the  alternating  day  and  night,  it  is  very  interesting  to  en- 
quire to  what  extent  Venus  may  in  this  respect  also  resemble 
our  globe.  Of  course  that  half  of  the  planet  Venus  which 
happens  to  be  turned  towards  the  Sun  revels  in  the  glory  of 
the  strong  sunbeams,  while  the  other  half  of  the  planet, 
necessarily  averted  from  the  Sun.  experiences  the  gloom  of 
night.  We  desire  to  know  whether  Venus  rotates  on  its 
axis,  and  if  it  does  under  what  conditions  its  rotations  are 
performed.  Only  when  these  questions  have  been  answered 
can  we  form  any  notion  as  to  what  may  be  the  law  of  the 


Mercury  and  Venus  101 

succession  of  day  and  night  on  Venus,  or  whether,  in  fact, 
there  is  any  such  succession  $.t&  particular  locality  on  the 
planet.  \  I  \'*j.  '  "./  \  ;  -j 

Astronomers  have  accordingly  devotee!  much  attention 
to  the  careful  scrutiny  of  the  plaaftt -^tih^ibji  tt&w  of 
discovering  in  what  way  it  turns  round  on  its  axis.  I  may 
here  say  that  the  difficulties  connected  with  this  research 
are  so  great  that  even  at  this  moment  we  cannot  feel 
confident  that  they  have  been  completely  overcome.  The 
fact  is,  that  Venus  presents  to  our  telescope  a  bright  and 
shining  globe  so  unsullied  by  any  conspicuous  marks  or 
spots  that  it  is  very  difficult  for  us  to  study  the  rotation  of 
its  globe.  If  there  were  on  the  surface  of  the  planet  even 
one  distinct  and  definite  mark,  which  could  be  unmistak- 
ably recognised  in  our  great  telescopes  when  the  side  of  the 
planet  on  which  it  lay  was  properly  placed,  then  by  the 
study  of  the  successive  appearances  of  this  spot,  as  it  was 
brought  into  view  by  the  planet's  rotation,  we  should  be 
able  to  learn  how  many  hours  the  planet  required  to  perform 
a  revolution.  But  unfortunately  Venus  bears  on  its  fail- 
surface  no  conspicuous  marks  of  the  kind  required.  There 
are  only  certain  very  faint  features  which  can  occasionally 
be  discerned  with  more  or  less  accuracy.  Our  information 
as  to  the  rotation  of  the  planet  is  therefore  derived  only 
from  somewhat  unsatisfactory  sources.  The  distinguished 
Italian  astronomer  Schiaparelli  has  paid  particular  atten- 
tion to  this  delicate  subject.  The  result  of  his  enquiries  is 
very  interesting,  and  indeed  so  unexpected,  that  it  may  be 
said  to  be  almost  startling.  He  has  shewn  it  to  be  highly 
probable  that  Venus  conducts  its  movements  in  such  a 
manner  as  always  to  present  the  same  face  to  the  Sun. 
Just  as  the  movements  of  the  Moon  are  so  arranged  that 
our  satellite  always  shews  the  same  face  to  the  Earth,  so 
it  would  now  seem  that  Venus  behaves  in  exactly  the 
same  manner  as  regards  the  great  central  luminary. 


102  Astronomy 

One  consequence  which  would  follow  from  this  motion 
should  be  specially  noted  here  in  connexion  with  the 
possibles  j3fcistence\  e'ff  life',  oil  the  sister  planet.  If  the 
rotation  of  the  planet* b'e  what  is  supposed  then  one  hemi- 
sphere miw3|  '4iiJ6y'#eype;tuiil  day  with  the  Sun  for  ever 
overhead.'  "On  such  'a*  world 'the  phenomena  of  sunrise  and 
sunset  would  be  equally  unknown.  Those  unhappy  regions 
which  are  situated  on  the  other  side  of  the  planet  must  for 
ever  endure  the  gloom  of  night.  It  is  obvious  under  these 
circumstances  that  if  there  be  life  on  Venus  then  that  life 
must  all  be  crowded  into  that  one  hemisphere  where  the 
sunshine  is  perennial.  Kegions  on  the  other  side  of  the 
planet  would  be  submitted  to  appalling  conditions  of  ever- 
lasting frost.  Living  forms  would  require  to  be  consider- 
ably modified  from  those  which  we  know  here,  ere  they 
could  be  adapted  to  dwell  in  a  blaze  of  perpetual  sun- 
beams. If  there  be  inhabitants  on  Venus  endowed  with 
curiosity  and  enterprise  akin  to  that  which  characterises  the 
inhabitants  of  our  Earth,  they  may  be  occasionally  tempted 
to  despatch  exploring  expeditions  from  their  sunny  climes 
to  discover  the  mysteries  of  their  dark  hemisphere,  just 
as  we  send  forth  expeditions  in  the  hope  of  solving  the 
mysteries  of  our  Arctic  and  Antarctic  regions. 

More  recently  however  an  ingenious  spectroscopic 
method  has  been  applied  to  the  investigation  of  the  rota- 
tion of  Venus.  Belopoiski  has  shewn  that  the  long  period 
of  rotation  which  Schiaparelli's  observations  seem  to  indi- 
cate is  probably  erroneous.  He  concludes  the  period  must 
be  a  short  one,  perhaps  comparable  with  the  length  of 
our  own  day.  The  question  is  still  not  finally  settled. 

Telescopic  observers  have  always  been  specially  struck 
with  the  extraordinary  brightness  of  Venus.  To  possess 
such  brightness  it  would  seem  that  the  globe,  in  so  far  as  we 
see  it,  cannot  be  composed  of  materials  resembling  those  of 
which  our  Earth's  surface  is  formed.  The  only  explanation 


Mercury  and  Venus  103 

that  we  are  able  to  give  is  that  Venus  is  probably  covered 
by  vast  clouds  and  from  the  upper  surface  of  these  clouds 
the  sunbeams  are  reflected.  It  is  thus  suggested  that 
water  must  be  present  on  this  planet,  for  we  do  not  know 
any  material  by  which  such  clouds  could  be  produced 
otherwise.  Acute  observers  have  sometimes  thought  that 
they  have  discerned  the  presence  of  caps  of  ice  and  snow 
at  the  poles  of  Venus,  just  as  there  are  similar  caps  of  ice 
and  snow  perennially  at  the  poles  of  our  Earth,  while 
there  are  also,  at  the  proper  seasons,  similar  ice  caps  on 
the  poles  of  Mars.  Evidence  has  been  collected  in  many 
ways  to  prove  that  there  is  an  atmosphere  around  Venus. 
We  cannot  indeed  assert  that  this  atmosphere,  either  in 
its  composition  or  in  its  abundance,  has  any  resemblance 
to  the  atmosphere  which  surrounds  our  Earth.  A  gaseous 
envelope  of  some  kind  does  however  most  certainly  enfold 
this  star. 


CHAPTER  VII 
MARS 

§  110.  Resemblance  to  the  Earth.  If  Venus  be  like 
the  Earth  in  bulk  and  mass  the  planet  Mars  in  physical 
features  still  more  nearly  resembles  our  globe.  Indeed 
the  study  of  this  neighbouring  planet  is,  from  every  point 
of  view,  of  extreme  interest. 

We  use  the  word '  neighbouring,'  however,  with  certain 
modifications.  Mars  is  certainly  not  the  Earth's  nearest 
neighbour.  That  position,  of  course,  belongs  to  the  Moon. 
And  even  among  the  planetary  hosts  it  is  known  that  an 
insignificant  little  asteroid  named  Eros  comes  at  certain 
seasons  much  closer  to  our  Earth  than  does  Mars.  As, 
however,  the  globe  of  Eros  is  too  minute  to  permit  us  to 
make  any  detailed  examination  of  its  surface,  this  asteroid 
offers  but  little  attraction  to  the  astronomer  when  viewed 
merely  as  a  telescopic  object. 

It  must  also  be  borne  in  mind  that  under  certain  con- 
ditions Venus  approaches  the  Earth  much  more  closely 
than  Mars  is  ever  permitted  to  do.  But  when  Venus  comes 
closest  to  our  view,  it  happens  that  it  is  then  not  at  all 
suitably  placed  for  observation.  The  telescopic  pictures  of 
it  are  quite  lacking  in  the  fulness  of  detail  which  we  would 
so  much  like  to  see.  On  the  other  hand,  when  Mars  comes 
104 


Mir*  105 

into  the  most  favourable  position  for  our  inspection,  though 
he  still  remains  at  the  not  inconsiderable  distance  of  thirty- 
live  million  miles,  he  admits  of  being  scrutinised  with  very 
considerable  satisfaction  to  the  observer.  Mars  is  not. 
however,  very  frequently  to  be  found  in  such  a  position. 
The  distance  of  this  planet  from  the  Earth  varies  from 
one  year  to  another,  and  consequently  whenever  it  does 
so  happen  that  Mars  approaches  specially  near,  a  vigorous 
effort  is  made  to  utilise  the  occasion  to  the  utmost.  AYe 
have  already  indicated  the  special  reason  why  this  partic- 
ular planet  is  in  many  ways  far  more  attractive  than  any 
of  the  others.  It  is  the  most  world-like  of  the  celestial  bod- 
ies that  are  known  to  us.  In  speaking  of  the  Sun  or  of  the 
Moon  it  is  always  the  contrast  between  their  structure 
and  that  of  a  globe  like  our  own  Earth  that  receives  at- 
tention and  illustration.  Quite  otherwise  is  it  with  respect 
to  Mars.  The  many  striking  points  in  which  Mars  resem- 
bles the  Earth  then  specially  attract  our  notice.  First  as 
to  the  simple  element  of  dimensions.  It  is  true  that  Mars 
is  a  good  deal  smaller  than  the  Earth,  inasmuch  as  the 
diameter  of  the  planet  is  less  than  half  that  of  our  globe. 
Still,  considering  that  Jupiter  has  a  diameter  more  than 
ten  times  as  great  as  that  of  the  Earth  and  a  volume  more 
than  twelve-hundred  times  larger,  we  may  naturally  regard 
a  planet  like  Mars  as  being  nearly  akin  to  us  in  the  solar 
system,  so  far,  at  least,  as  bulk  is  concerned. 

§  111.  Surface  Markings.  It  is,  however,  in  the  actual 
details  of  the  surface  of  the  planet  that  the  resemblances 
between  this  globe  and  our  Earth  are  of  particular  inter- 
est. In  this  respect  the  observations  of  recent  years  are 
especially  worthy  of  attention.  There  can  be  no  doubt 
that  pictures  of  our  Earth,  if  drawn  by  an  observer  sta- 
tioned on  an  external  point,  would  be  specially  character- 
ised by  the  partition  of  the  whole  globe  into  surfaces  of 
land  and  surfaces  of  water,  as  well  as  by  the  circumstance 


106  Astronomy 

that  sea  and  land  are  alike  enveloped  by  a  vast  covering 
of  atmosphere.  On  the  surface  of  Mars  we  can  detect  ex- 
tensive tracts,  usually  of  a  more  or  less  ruddy  hue,  which  are 
generally  believed  to  be  continents  of  land,  or  rather  what 
\ve  may  describe  as  vast  deserts.  These  are  separated  from 
each  other  by  dark  regions,  in  some  cases  very  dark,  and  un- 
til recent  years  it  was  always  customary  to  regard  these 
dark  regions  as  indicating  oceans  of  water.  But  the  recent 
observations,  especially  those  of  Mr.  Percival  Lowell  at 
Flagstaff  Observatory  in  Arizona,  have,  I  think,  now  fully 
established  that  these  dark  regions  on  Mars  cannot  be  re- 
garded as  oceans.  Examined  under  the  favourable  circum- 
stances which  are  found  at  Mr.  Lowell's  observatory,  definite 
marks  can  be  discerned  through  these  dark  areas  which  are 
wholly  incompatible  with  the  supposition  that  such  tracts 
are  expanses  of  ocean.  It  now  seems  much  more  probable 
that  the  dark  regions  are  places  in  which,  owing  to  the  pres- 
ence of  water,  fertility  has  been  gi  ven  to  the  soil.  The  con- 
trast between  these  dark  tints  and  the  ruddy  hues  of  the 
other  parts  of  the  planet  would  be  explained  by  admitting 
that  the  latter  are  deserts,  devoid  of  vegetation  or  water. 
In  fact,  the  whole  surface  of  the  planet  seems  to  possess  noth- 
ing that  can  be  described  as  an  ocean  of  water,  or  even  as  a 
sea.  Water  seems  to  be  a  very  scarce  element,  and  in  con- 
nexion with  it  Mr.  Lowell  has  made  some  suggestions  as 
to  the  probability  of  the  existence  of  inhabitants  of  Mars 
which  it  must  be  admitted  are  borne  out  to  some  extent  by 
the  observations  which  he  has  succeeded  in  making  (§  116). 
§  112.  Polar  Caps.  At  the  season  when  Mars  makes 
its  closest  approach  to  the  Earth  there  is  no  more  interesting 
telescopic  object  in  the  solar  system  than  the  polar  regions 
of  that  planet.  The  north  pole  or  the  south  pole,  which- 
ever of  the  two  may  happen  to  be  visible,  is  distinctly  seen 
to  be  covered  with  a  definite  mass  of  white  material.  It  is 
impossible  not  to  believe  that  this  white  polar  cap  is  a  mass 


107 

of  ice  or  snow.  If  we  were  able  to  take  such  a  bird's-eye 
view  of  our  own  globe  as  we  obtain  of  Mars  under  the 
circumstances  mentioned,  we  should  find  around  the  Pole 
an  accumulation  of  ice  and  snow  which  would  give  to  the 
neighbourhood  just  the  same  aspect  as  that  which  Mars 
presents  to  us.  The  evidence  that  these  Martian  caps  are 
composed  of  snow  becomes  very  much  strengthened  when 
we  notice  that  their  limits  are  not  invariable.  Sometimes 
the  white  mass,  whatever  it  may  be,  advances  to  lower 
latitudes  of  the  planet,  sometimes  it  retreats  so  as  to 
withdraw  more  closely  to  the  Pole,  and  sometimes  it 
vanishes  altogether. 

It  should  also  be  noticed  that  Mars  has  seasons  of 
Summer  and  seasons  of  Winter.  They  are  of  course  in  no 
Avay  related  to  our  seasons  which  are  designated  by  the 
same  names.  But  in  virtue  of  the  movement  of  the 
planet  around  the  Sun  and  of  the  inclination  of  its  axis 
to  the  plane  of  its  orbit,  it  is  obvious  that  there  must  be 
a  winter  and  a  summer  on  Mars  contrasted  in  much  the 
same  way  as  the  seasons  are  here.  When  it  is  summer  in 
the  Northern  hemisphere  of  Mars,  it  will  be  winter  in  the 
Southern  hemisphere  on  the  same  globe,  and  vice  versd. 
Careful  observations  have  demonstrated  the  significant 
circumstance  that  during  the  Martian  summer  the  icy 
polar  mass  or,  at  all  events,  the  white  mass,  is  observed 
to  decline  and  shrink,  while  during  the  winter  it  again 
enlarges  and  occupies  once  more  the  neighbouring  regions. 
In  some  cases  the  retreating  sheet  leaves  behind  it  frag- 
mentary portions  isolated  on  mountain  summits,  and  these 
Income  reunited  with  the  main  mass  when  in  the  course 
of  time  the  season  of  extension  has  again  returned.  Con- 
sidering the  analogy  of  our  own  Earth,  and  all  the 
circumstances  of  the  case,  it  is  hardly  reasonable  to 
refuse  our  assent  to  the  belief  that  what  we  are  looking 
at  must  be  indeed  a  vast  cap  of  ice  and  snow  covering 


108  Astronomy 

the  polar  regions  of  the  planet.  If  this  is  admitted  its 
existence  forms  a  very  important  piece  of  testimony  in 
favour  of  the  doctrine  that  water  exists  on  Mars. 

§  113.  Atmosphere.  It  no  longer  admits  of  any 
question  that  this  planet  which  is  already  so  like  our 
Earth  in  the  respects  we  have  just  named  is  also  like  our 
Earth  in  the  possession  of  an  atmosphere.  Eecent  ob- 
servations have  left  no  room  for  doubt  on  this  point.  It 
must,  however,  be  acknowledged  that  the  atmosphere 
round  the  Earth  is  far  more  voluminous  than  that  around 
Mars.  Indeed  it  seems  very  doubtful  whether  it  would  be 
possible  for  an  observer,  looking  at  our  Earth  from  the 
same  distance  at  which  we  have  to  look  at  Mars,  would 
be  able  to  obtain  any  very  clear  idea  as  to  positions  of  the 
great  oceans  or  the  trend  of  the  various  continents.  The 
obstruction  offered  by  the  atmosphere  of  our  Earth  is  so 
considerable  that  there  would  be  great  difficulty  in  seeing 
anything  clearly  on  its  surface.  It  is  at  all  events  quite 
certain  that  we  are  able  to  distinguish  the  several  features 
on  Mars  far  more  clearly  than  any  inhabitant  of  Mars 
would  be  able  to  distinguish  the  features  of  our  Earth. 
This  consideration  at  once  leads  to  the  conclusion  that  the 
atmosphere  which  surrounds  the  ruddy  planet  must  be 
much  less  copious  than  the  atmosphere  on  our  Earth,  at 
the  bottom  of  which  we  reside.  Here  then  is  an  important 
distinction  between  Mars  and  the  Earth  in  so  far  as  the 
quantity  of  its  atmosphere  is  concerned.  There  remains, 
however,  a  very  important  question  as  to  the  nature  of  the 
gases  which  compose  the  Martian  air.  This  would  of 
course  be  a  vital  question  so  far  at  least  as  the  relations 
of  the  Martian  inhabitants  is  concerned.  We  do  not, 
however,  possess  at  present  any  adequate  information  on 
this  important  matter.  There  is  some  reason  to  think  that 
the  gases  in  the  atmosphere  of  Mars  are  not  wddely 
different  from  the  gases  which  compose  our  own  atmos- 


Jfars  109 

phere.  But  it  is  very  unlikely  that  the  proportions  in 
which  the  different  gases  are  mixed  should  be  at  all 
similar  in  the  two  bodies. 

Among  many  interesting  observations  that  have  been 
made  of  Mars  in  recent  times  we  shall  first  specially  notice 
the  work  of  Monsieur  Perrotin.  He  has  minutely  studied 
the  atmosphere  of  the  planet  with  the  grand  telescope  at 
Nice  in  the  south  of  France,  and  he  has  added  a  good 
many  interesting  facts  to  our  knowledge. 

§  114.  The  Canals.  Monsieur  Perrotin  has  also  se- 
cured some  valuable  observations  which  may  be  regarded 
as  having  finally  set  at  rest  certain  points  in  the  physical 
geography  of  Mars  which  have  long  been  in  dispute.  It  is 
now  many  years  since  the  distinguished  Italian  astronomer 
Professor  Schiaparelli  announced  that  the  continents  of 
Mars  were  in  many  places  traversed  from  coast  to  coast  by 
long  straight  streaks.  From  the  analogy  suggested  by  their 
appearance  he  designated  these  objects  as  canals.  It  may 
illustrate  the  difficulty  and  the  delicacy  of  such  observa- 
tions Avhen  we  add  that  certain  other  astronomers  of 
acknowledged  skill,  and  provided  with  excellent  instru- 
ments, have  not  succeeded  in  detecting  these  remarkable 
features.  Perrotin,  however,  adds  the  weight  of  his  high 
authority  to  confirm  the  observation  of  Schiaparelli.  He 
says  that  although  at  the  time  he  made  his  observation 
the  conditions  were  not  quite  so  favourable  as  might  be 
desired  for  the  examination  of  some  of  the  canals,  yet 
certain  of  them  were  wholly  unmistakable,  and  in  allud- 
ing to  the  controversy,  he  adds,  that  some  of  these  were 
so  easy  to  recognise  that  even  the  most  prejudiced  observer 
must  have  been  convinced. 

§  115.  Changes  in  the  Great  Syrtis.  At  the  same 
observatory  special  pains  had  been  given  to  the  study 
of  the  remarkable  tract  on  Mars  which  Perrotin  desig- 
nates as  the  Great  Syrtis.  This  used  to  be  regarded  as  a 


110  Astronomy 

sea  or  ocean,  but  according  to  our  present  knowledge  it 
should  rather  be  regarded  as  a  tract  in  which  the  pres- 
ence of  water  is  sufficient  to  render  vegetation  possible. 
Perrotin  has  compared  together  the  drawings  of  the  Great 
Syrtis  made  at  different  times,  and  it  has  been  distinctly 
found  that  changes  in  the  outline  of  this  remarkable 
feature  can  be  discerned.  The  view  which  Perrotin  takes 
is  that  mists  and  clouds  though  prevailing  on  Mars  in 
a  lesser  degree  than  they  do  here,  are  still  at  times 
sufficiently  copious  to  obscure  our  view,  and  to  hide  some 
of  the  canals  traversing  the  northern  regions  near  the 
Great  Syrtis. 

§  116.  Lowell's  Theory.  These  canals,  which  were  dis- 
covered by  Schiaparelli  and  whose  existence  was  con- 
firmed by  Perrotin,  have  also  been  studied  by  Mr.  Percival 
Lowell.  The  view  which  he  puts  forward  is  connected 
with  the  remarkable  circumstance  now  certainly  estab- 
lished that  there  are  no  wide  seas  or  oceans  on  Mars.  Water 
there  is  on  the  planet,  but  that  water  is  not  of  sufficient 
abundance  to  form  great  sheets  such  as  occupy  so  large  a 
part  of  our  Earth.  The  consequence  is  that  if  there  be 
inhabitants  on  Mars  the  provision  of  the  supplies  of  water 
necessary  for  their  welfare  is  a  matter  involving  skilled 
organisation.  Each  winter  of  that  planet  sees,  as  I  have 
explained,  the  accumulation  of  a  cap  of  ice  and  snow 
around  the  corresponding  Pole.  In  the  course  of  the 
ensuing  summer  that  icy  mass,  or  at  any  rate  a  very  great 
portion  of  it,  is  restored  to  the  form  of  water.  Mr.  Lowell 
has  detected  around  the  Poles  of  Mars  what  he  believes  to 
be  the  masses  of  water  that  have  been  thus  accumulated. 

§  117.  Rotation.  The  definiteness  of  the  markings  on 
Mars  makes  it  a  comparatively  easy  matter  to  determine 
the  time  in  which  it  performs  a  rotation  on  its  axis  or 
the  length  of  its  day.  In  this  particular,  too,  we  find 
that  this  planet  closely  resembles  the  Earth,  as  it  takes 


Mars  111 

only  about  41  minutes  longer  to  rotate  than  does  the 
Earth.  The  exact  length  of  its  day  is  24  hrs.  37  mins. 
22f  sees. 

§  118.  Satellites.  Up  to  the  year  1877,  Mars  was 
believed  to  be,  like  Venus,  unattended  by  any  satellites ; 
but  in  that  year,  taking  advantage  of  the  fact  that  Mars 
approached  unusually  close  to  the  Earth,  Prof.  Asaph  Hall 
discovered  two  minute  moons  attending  the  planet. 

The  outer  of  these  satellites  revolves  around  the  planet 
in  the  period  of  30  hrs.  17  mins.  54  sees. ;  it  is  the  inner 
satellite  which  has  commanded  the  attention  and  curiosity 
of  every  astronomer  in  the  world.  The  inner  satellite  of 
Mars  moves  round  in  7  hrs.  39  mins.  14  sees. !  It,  in  fact, 
revolves  three  times  round  Mars  in  the  same  time  that 
Mars  can  turn  round  once.  This  circumstance  is  without 
a  parallel  in  the  Solar  System  ;  indeed  so  far  as  we  know 
it  is  unparalleled  in  the  Universe.  There  is  no  other 
known  case  where  a  satellite  revolves  around  its  primary 
more  quickly  than  the  primary  rotates  on  its  axis. 

Both  of  these  bodies  are  extremely  minute.  Deimos, 
the  outer,  in  all  probability  does  not  exceed  18  miles  in 
diameter,  while  Phobos,  the  inner  one,  can  scarcely  ex- 
ceed 23  miles  in  diameter. 


CHAPTER   VIII 

THE  ASTEROIDS 

§  119.  Number  of  Asteroids.  Nearly  five  hundred 
comparatively  small  globes  which  we  call  Asteroids  are 
now  known  to  belong  to  the  solar  system.  The  planets  so 
designated  may  be  described  as  small  when  we  think  of 
the  robust  dimensions  of  our  Earth  or  even  of  the  Moon. 
They  would  however  be  by  no  means  unimportant  when 
judged  by  certain  other  standards.  The  surfaces  of  some 
of  the  minor  planets  might  no  doubt  not  be  large  enough 
to  contain  an  area  greater  than  that  of  London,  but  on 
the  other  hand  some  of  them  would  not  be  too  small  to 
contain  the  whole  of  Great  Britain.  The  position  of  the 
zone  of  the  Asteroids  is  indicated  in  Fig.  21,  which  shews 
the  solar  system  exterior  to  the  orbit  of  Mars. 

§  120.  How  an  Asteroid  is  distinguished  from  a 
Star.  Owing  to  their  small  size  and  their  distance  from 
the  Earth  the  minor  planets  are  almost  always  invisible 
to  the  unaided  eye.  They  are  only  to  be  observed  with 
the  help  of  the  telescope.  But  though  the  more  important 
of  the  Asteroids  may  be  quite  bright  enough  to  be  readily 
seen  by  any  one  who  is  using  a  good  telescope,  yet  the 
observer  will  generally  find  it  is  by  no  means  easy  to 
discriminate  the  planet  on  which  he  wants  to  fix  his 
112 


The  Asteroids  ll.'J 

attention  from  among  the  small  stars  which  are  so  pro- 
fusely strewn  around  the  neighbouring  parts  of  the  heavens. 
>Jo  doubt  as  a  matter  of  fact  there  is  the  profoundest  dif- 
ference between  the  actual  nature  of  the  planet  and  that 
of  a  star.  The  latter  is  a  sun-like  object,  generally  millions 
of  times  bigger  and  hundreds  of  millions  of  times  brighter 
than  the  planet.  But  far  from  the  Earth  as  the  planet 
may  be,  the  stars  are  millions  of  times  further  still.  It 
follows  from  this  consideration  that  the  intrinsic  splendour 
of  the  star  is,  when  viewed  from  our  point  of  view,  reduced 
to  a  feeble  twinkle,  a  twinkle  so  like  the  faint  rays  emitted 
from  the  planet  that,  so  far  as  mere  appearance  goes,  it 
would  be  impossible  to  decide  by  a  glance  through  the 
telescope  as  to  which  was  the  sun-like  object  and  which 
was  the  earth-like  object.  The  test  by  which  we  decide 
between  a  planet  and  a  star  is  to  be  sought  in  the  circum- 
stance that  the  planet  is  in  motion,  while  the  star  remains 
fixed.  We  may  at  least  regard  the  star  as  fixed  in 
comparison  with  the  movements  of  the  planet  which  are 
apparently  thousands  of  times  as  great.  It  is  therefore  by 
making  an  assiduous  search  through  the  heavens  for  the 
little  star-like  points  which  are  in  motion  that  the  dis- 
covery of  the  minor  planets  is  to  be  effected. 

§  121.  Photographic  Methods.  The  astronomer  is  now 
able  to  avail  himself  of  a  new  and  greatly  superior  method 
by  which  the  little  planet  can  be  made  to  betray  its  ex- 
istence. If  a  photographic  plate  is  placed  in  the  telescope, 
furnished  with  an  accurate  clock-work  motion  which  will 
keep  it  pointing  to  the  same  part  of  the  sky,  and  if  an  expos- 
ure of  an  hour  or  so  be  given  to  the  plate,  then  as  each  of  the 
stars  has  not  moved  it  will  record  itself  as  a  point  on  the 
developed  picture.  If,  however,  it  should  have  happened 
that  a  planet  was  situated  at  the  time  in  the  part  of  the  sky 
which  is  represented  on  the  plate,  then  as  this  object  is 
in  motion  it  will  not  form  a  dot  like  a  star,  it  will  rather 


114  Astronomy 

manifest  its  presence  by  a  streak  instead  of  a  point.  In 
this  way  the  examination  of  a  plate  produced  by  long 
exposure  will  enable  the  astronomer  to  determine  whether 
among  the  numerous  star-like  points  scattered  over  the 
part  of  the  sky  which  he  has  been  studying  there  should 
happen  to  lie  one  of  these  wandering  planets. 

122.  Size.  Like  the  greater  planets,  the  Asteroids 
are  generally  named  after  classical  nymphs  or  divinities. 
I  may,  for  instance,  mention  one  of  the  most  interesting 
which  is  known  as  Vesta,  the  name  given  to  it  when  it 
was  discovered  by  Olbers  in  the  year  1807.  Though  Vesta 
is  one  of  the  most  considerable  of  the  minor  planets  yet 
viewed  in  the  telescope  it  is  rather  a  disappointing  object. 
It  is  generally  speaking  no  more  than  a  star-like  point ; 
indeed  it  requires  a  specially  excellent  telescope  and  other 
favouring  circumstances  to  shew  that  Vesta  possesses  a 
perceptible  circular  outline.  In  a  small  telescope  Vesta  is 
merely  a  point  exhibiting  no  more  disc  than  a  star.  The 
measurement  of  the  diameter  of  such  an  object  is  therefore 
a  problem  of  no  little  difficulty.  It  has,  however,  been 
determined  by  Professor  E.  E.  Barnard  who  used  for  this 
purpose  the  great  telescope  of  the  Lick  Observatory.  He 
thus  ascertained  that  the  diameter  of  Vesta  is  243  miles. 
Most  of  the  other  minor  planets  are  much  smaller  than  this 
little  globe.  Indeed  the  great  majority  of  Asteroids  are  so 
small  as  to  elude  direct  measurement  altogether.  \Ve  can 
do  little  more  than  form  conjectural  estimates  as  to  their 
dimensions.  It  seems,  however,  pretty  certain  that  several 
of  these  objects  are  so  insignificant  as  to  have  a  diameter 
of  not  more  than  ten  miles. 

§  123.  Zone  of  Minor  Planets.  Discovery  of  Eros. 
The  Asteroids  revolve  in  the  region  of  the  Solar  System 
which  is  included  between  the  track  followed  by  Mars 
and  the  track  followed  by  Jupiter  (see  Fig.  21,  p.  119). 
Such  is,  at  least,  the  region  within  which  most  of  these 


Tlie  Asteroids  115 

objects  are  confined.  But  there  are  a  few  of  the  minor 
planets  which  at  one  part  of  their  path  come  within  the 
track  of  Mars  and  which  at  another  part  extend  out  to 
that  of  Jupiter.  A  discovery  has  lately  been  made  by 
Witt  of  Berlin,  of  the  Asteroid  which  bears  the  number 
433,  and  to  which  has  been  given  the  title  of  Eros.  This 
object  is  quite  a  small  one  and  would  be  of  no  particular 
interest  except  for  one  circumstance.  This  circumstance 
however  makes  it  so  remarkable  that  it  is  certainly  not 
too  much  to  say  that  Eros  is  of  greater  importance  in 
Astronomy  than  all  the  rest  of  the  Asteroids  put  together. 
It  so  happens  that  the  track  of  Eros  lies  within  the  tracks 
which  the  other  Asteroids  pursue,  so  much  so  that  on  cer- 
tain occasions  Eros  approaches  the  Earth  to  a  distance 
not  exceeding  thirteen  million  miles.  This  may,  no  doubt, 
seem  a  large  figure  regarded  from  some  points  of  view, 
but  judged  by  the  scale  of  distances  in  the  Solar  System, 
it  is  extremely  small.  Setting  aside  the  Moon  which  is 
of  course  merely  an  appendage  to  the  Earth,  it  appears 
that  Eros  comes  closer  to  the  Earth  than  any  other  globe 
in  the  Universe.  Venus  and  Mars  used  formerly  to  be 
spoken  of  as  the  Earth's  neighbours,  but  now  these  large 
planets  have  to  yield  the  position  of  being  the  Earth's 
nearest  planetary  neighbour  to  this  little  globe  Eros,  which 
is  so  insignificant  in  dimensions  that  one  hundred  millions 
of  them  taken  together  would  not  be  so  large  as  Venus. 

$  124.  Importance  of  Eros.  The  importance  of  Eros 
to  the  astronomer  can  hardly  be  overestimated.  It  fol- 
lows from  Kepler's  laws,  which  we  have  explained  in 
Chap.  V.,  that  the  squares  of  the  periodic  times  of  the 
planets  are  proportional  to  the  cubes  of  their  mean  dis- 
tances from  the  Sun.  We  know  all  the  periodic  times 
accurately.  This  element  indeed  admits  of  being  deter- 
mined with  extreme  precision,  and  as  we  know  the  peri- 
odic times  we  can  infer  from  this  remarkable  law  of  Kepler 


116 

the  proportionate  values  of  the  mean  distances.  We  thus 
know  with  all  desirable  precision  the  relative  values  of 
the  diameter  of  the  orbit  of  Venus  and  of  the  orbit  of  the 
Earth ;  we  know  also  how  many  times  the  mean  distance 
of  the  Earth  is  contained  in  the  mean  distance  of  Jupiter 
or  of  Saturn  or  of  Xeptuue.  Not  only  are  the  relative 
values  of  the  distances  of  these  bodies  thus  determined, 
but  the  proportion  of  the  size  of  the  bodies  to  their  dis- 
tances is  also  involved.  We  thus  know,  for  instance,  the 
ratio  which  the  Sun's  diameter  bears  to  the  mean  distance 
of  the  Earth  from  the  Sun.  Hence  it  follows,  speaking  now 
quite  generally,  that  we  know  the  relative  values  of  the 
chief  dimensions  in  the  Solar  System.  But  that  knowledge 
is  not  sufficient.  We  further  desire  to  know  the  actual 
values  of  all  these  magnitudes.  This  information  will  be 
given  if  we  learn  any  one  of  the  distances.  If,  for  instance, 
we  have  been  able  to  find  by  any  direct  observations  the 
distance  of  the  Earth  from  the  Sun,  then  with  the  help  of 
our  known  ratios  we  are  able  to  find  the  mean  distance  of 
Venus  from  the  Sun  or  the  mean  distance  of  Jupiter  from 
the  Sun.  We  are  also  able  to  find  the  value  of  the  Sun's 
diameter.  In.  like  manner  if  observations  have  taught 
us  the  distance  of  Mars  from  the  Earth,  then  by  apply- 
ing the  same  principle  of  Kepler  we  shall  be  enabled  to 
ascertain  the  distance  of  Mars  from  the  Sun  and  the  dis- 
tances of  the  other  planets.  All  our  accurate  knowledge 
depends  therefore  on  obtaining  a  precise  determination  of 
the  distance  of  one  object  in  the  Solar  System.  This  is 
always  a  problem  of  great  difficulty.  The  difficulty  arises 
from  the  fact  that  even  the  nearest  object  to  us  (the  Moon 
is  again  excepted)  is  still  very  remote.  We  therefore  look 
to  the  planets  which  come  nearest  to  us  to  help  in  the 
solution  of  this  problem.  Venus,  for  instance,  is  at  its 
closest  approach  when  it  passes  directly  between  us  and  the 
Sun.  This  phenomenon — known  as  a  Transit  of  Venus  —  is 


TJie  Aitti'i-oiil*  117 

of  very  rare  occurrence,  but  when  it  does  happen  it  affords 
certain  facilities  for  determining  the  distance  of  Venus 
\vhich  have  been  utilised  for  this  important  piece  of  sur- 
veying work.  On  other  occasions  the  opportunity  has  been 
taken  when  Mars  is  found  in  its  position  of  least  distance 
from  the  Earth,  and  its  distance  has  been  measured.  Now 
however  the  discovery  of  Eros  has  given  to  astronomers  an 
object  much  more  suited  for  this  investigation  than  either 
Mars  or  Venus.  The  planet  Eros  when  it  comes  nearest  is 
about  half  the  distance  of  Venus  when  nearest,  and  at  all 
times  Mars  is  at  least  three  times  as  far  from  us  as  is  Eros 
when  the  right  moment  comes.  Further,  the  little  point 
which  Eros  presents  to  us  in  the  telescope  is  one  which  will 
admit  of  extremely  accurate  measurement.  For  such  work 
the  astronomer  uses  what  is  called  the  micrometer,  and  in 
the  best  known  type  of  micrometer  fine  lines,  which  are 
really  the  lines  spun  by  the  spider,  are  stretched  across  the 
field  of  view  and  are  mounted  in  frames  which  are  moved 
by  screws  and  admit  of  the  most  accurate  measurements. 
The  measurements  of  a  little  point  like  Eros  from  the 
adjacent  points  which  represent  stars  can  be  made  with 
the  highest  precision  known  in  astronomical  art,  and  from 
such  measurements,  made  simultaneously  at  two  widely 
separated  observatories,  the  distance  of  the  little  planet 
may  be  deduced.  We  thus  look  to  Eros  as  affording  the 
means  by  which  the  scale  on  which  the  Solar  System  is 
constructed  may  be  best  determined. 

§  125.  How  to  determine  the  Distance  of  a  Minor 
Planet.  The  ad  join- 
ing figure  will  explain 
the  nature  of  the  obser- 
vations which  have 
to  be  made.  Ity  pre- 
vious agreement  be- 
tween two  astrono-  Fi--- 2a  ^tance  of  Eros. 


118  Astronomy 

iners,  situated  in  observatories  a  long  distance  apart,  say, 
for  instance,  at  Greenwich  and  at  the  Cape  of  Good  Hope, 
the  apparent  angular  distance  between  Eros  and  a  neigh- 
bouring star  is  measured.  As  the  stars  are  at  what  is 
practically  an  infinite  distance  they  may  be  regarded  as 
being  in  the  same  direction  when  viewed  from  Greenwich 
or  from  the  Cape  of  Good  Hope,  which  are  represented 
in  the  figure  by  the  letters  G  and  C.  But  Eros  is  com- 
paratively so  close  that  its  distance  from  the  Earth  bears 
quite  an  appreciable  ratio  to  the  distance  between  the  two 
observatories.  The  consequence  is  that  the  place  of  the 
planet  as  observed  from  the  northern  observatory  is  dis- 
tinctly different  from  its  place  viewed  from  the  southern 
observatory.  Thus  the  two  base  angles  of  the  triangle 
whose  base  is  the  line  joining  the  two  observatories  and 
whose  vertex  is  at  the  planet  are  measured  and  then  by 
the  operations  of  Trigonometry  the  distance  of  the  planet 
is  calculated. 

The  process  I  have  described  has  already  been  applied 
with  some  success  to  certain  other  Asteroids.  But  when 
Eros  is  most  favourably  placed  it  will  be  at  least  three  or 
four  times  as  suitable  as  the  other  Asteroids  have  been. 
We  usually  express  these  distances  in  terms  of  the  Sun's 
distance,  and  therefore  we  may  speak  of  Eros  as  giving 
the  distance  of  the  Sun.  To  the  best  of  our  present  know- 
ledge the  distance  of  the  Sun  may  be  now  represented  as 
92,900,000  miles.  In  the  present  state  of  Astronomy  it 
ought  now  to  be  possible  to  determine  this  fundamental 
element  with  an  accuracy  which  shall  certainly  be  within 
one-thousandth  part  of  the  whole. 


CHAPTER  IX 
JUPITER 

THE  four  great  outer  planets  of  our  system  revolve  in 
orbits  of  which  the  relative  dimensions  are  shewn  in  the 
adjoining  figure.  Jupiter  completes  its  revolution  in  11-9 


Fig.  21.     The  Outer  Planets. 

years,  while  Saturn,  Uranus  and  Neptune  require  re- 
spectively 29-5,  84  and  164-6  years.       In  the  present 
119 


Jupiter  121 

round  on  his  axis  with  amazing  rapidity  when  the  size  of 
his  mighty  globe  is  taken  into  consideration.  The  time 
that  he  requires  for  each  rotation  is  only  about  ten  hours. 
This  may  naturally  be  contrasted  with  the  period  of  almost 
twenty-four  hours  which  our  Earth  requires  for  the  same 
purpose.  If  it  be  further  remembered  that  the  diameter 
of  Jupiter  is  about  ten  times  that  of  the  Earth,  it  will 
be  obvious  that  the  strain  arising  from  the  centrifugal 
force  on  the  equatorial  regions  of  Jupiter  must  be  greatly 
in  excess  of  the  analogous  strain  on  the  Earth.  We  need 
therefore  no  longer  feel  any  surprise  that  the  great  planet- 
should  exhibit  in  such  a  striking  manner  its  departure 
from  a  circular  outline. 

$  128.  Belts.  The  chief  features  to  be  observed  on 
the  disc  of  Jupiter  are  the  remarkable  dark  belts  which 
encircle  the  planet  parallel  to  its  equator,  and  they  merit 
special  attention  because  they  exhibit  a  state  of  things 
unlike  what  is  found  elsewhere  in  the  Solar  System.  A 
little  careful  observation  will  shew  that  these  belts  are  not 
constant  in  appearance,  they  are  sometimes  much  more 
amply  developed  than  at  other  times.  Sometimes  they 
change  their  places  or  become  broken  up,  while  on  rare 
occasions  they  disappear  altogether. 

£  129.  Changes  in  the  Appearance  of  the  Belts.  A  con- 
siderable proportion  of  the  changes  noticed  may  of  course 
be  attributed  to  the  rotation  of  the  planet.  For  as  Ave 
have  already  mentioned,  this  body  completes  a  rotation  in 
at  Hint  ten  hours.  It  therefore  follows  that  after  a  period 
of  five  hours  the  planet  will  have  turned  through  half  a 
revolution,  and  a  complete  transformation  will  have  been 
effected  in  the  hemispheres  presented  for  our  inspection. 
In  other  words,  if  the  astronomer  looks  at  Jupiter  at  eight 
o'clock  in  the  evening,  and  turns  his  telescope  to  the  planet 
again  at  one  o'clock  in  the  course  of  the  same  night,  there 
is  not  a  single  part  of  the  surface  which  was  visible  to  him 


H'l'  Astronomy 

on  the  first  occasion  within  view  on  the  second.  Of  course 
this  causes  much  change  in  the  appearance  of  any  feature 
on  Jupiter  due  to  the  varied  foreshortening  that  it  under- 
goes. But  when  full  allowance  has  been  made  for  all 
variations  in  the  attitude  of  the  planet  which  arise  from 
its  rotation  it  still  remains  perfectly  certain  that  all  the 
changes  in  the  belts  cannot  be  thus  accounted  for.  It  is 
certain  that  these  belts  cannot  be  regarded  as  permanent 
settled  features  of  the  globe.  If  this  were  the  case,  then 
by  waiting  for  ten  hours,  or  for  any  entire  number  of  the 
periods  of  rotation,  we  should  always  find  that  the  face  of 
the  planet  turned  towards  us  was  marked  in  exactly  the 
same  way.  But  nothing  of  the  kind  is  found  to  take  place. 
There  are  sometimes  changes  sufficiently  marked  to  be 
perceptible  in  the  course  of  even  a  single  rotation  of  the 
globe.  It  is  thus  plain  that  the  belts  and  similar  features 
on  Jupiter  cannot  be  permanent  objects  engraved  on  his 
mighty  sphere ;  they  must  be  counted  as  comparatively 
transient  and  unstable. 

§  130.  Clouds.  A  glance  at  the  clouds  in  our  own 
atmosphere  suggests  at  once  what  is  indeed  the  true 
solution  of  the  varied  appearances  presented  by  the  great 
planet.  It  is  obvious  that  Jupiter  must  be  enveloped  in 
clouds,  and  it  is  these  clouds  which  are  presented  to  us 
when  we  look  at  the  planet  through  our  telescopes.  The 
characteristic  features  of  the  planet  can  be  accounted  for 
on  this  supposition,  and  here  we  are  assisted  by  further 
analogies  suggested  by  our  own  atmosphere.  The  fact  that 
the  Jovian  clouds  are  mainly  arranged  in  belts  parallel  to 
the  equator  of  the  planet  calls  to  mind  at  once  the  analo- 
gous manner  in  which  we  find  cloud  belts  on  the  Earth 
parallel  to  the  terrestrial  equator.  The  trade  winds  are 
well  known  to  be  connected  with  these  equatorial  zones 
and  to  determine  the  positions  of  corresponding  zones  of 
cloud  upon  the  Earth.  It  seems  not  improbable  that  our 


Jupiter  123 

globe  would  present  to  an  observer  who  was  viewing  it 
from  some  distance  in  space  an  aspect  somewhat  similar 
to  that  which  Jupiter  presents  to  us,  in  so  far  at  least  as 
the  disposition  of  clouds  on  its  surface  is  concerned. 

§  131.  Impenetrability  of  the  Clouds.  It  is,  however, 
to  be  noticed  that  the  clouds  on  the  great  planet  are 
much  more  copious  than  are  the  clouds  on  the  Earth. 
I  do  not  now  mean  merely  that  because  Jupiter  has  an 
area  more  than  a  hundred  times  as  great  as  that  of  the. 
Earth,  therefore  the  quantity  of  clouds  which  encompass 
him  is  correspondingly  larger.  The  cloud  masses  on 
Jupiter  are  indeed  far  greater  than  in  this  numerical 
proportion.  Area  for  area,  Jupiter  is  covered  by  cloud 
masses  many  times  thicker  and  deeper  than  the  clouds  on 
the  Earth.  We  can  prove  this  by  the  simple  consideration 
that  our  clouds,  though  doubtless,  as  we  may  think,  often 
thick  enough,  are  still  not  always  present.  They  do  now 
and  then  disperse,  and  enable  us  to  obtain  a  view  of  the 
Sun.  If,  however,  there  should  be  any  inhabitants  on 
Jupiter  they  can  never  be  so  fortunate.  So  vast  is  the 
depth  of  the  mighty  Jovian  clouds  that  they  never  disperse 
sufficiently  to  allow  the  gaze  of  one  situated  on  the  surface 
below  them  to  pierce  through  to  outer  space.  Nor,  on  the 
other  hand,  will  the  clouds  on  Jupiter  permit  us,  who  are 
studying  the  globe  from  the  outside,  to  explore  the  depths 
of  that  Jovian  atmosphere  and  see  what  its  interior  globe 
is  like.  It  may  indeed  be  said  that  we  have  never  yet 
certainly  had  a  view  of  any  permanent  feature  whatever 
on  the  great  planet,  with  one  possible  exception  in  the 
famous  red  spot. 

§  132.  Storms.  From  the  size  of  the  clouds  on  Jupi- 
ter, and  the  rapidity  of  the  changes  they  execute,  it  is 
manifest  that  the  surface  of  the  great  globe  must  be  not 
(infrequently  swept  by  storms  and  tempests.  The  terrific 
vehemence  of  these  changes  could  never  be  accounted  for 


if  we  merely  invoked  the  same  agents  for  their  production 
which  are  so  effective  in  our  terrestrial  storms.  For  just 
see  how  the  matter  stands.  The  gales  with  which  our 
terrestrial  atmosphere  is  occasionally  distracted  are  ulti- 
mately due  to  the  action  of  the  sunbeams  by  which  those 
winds  are  raised.  But  Jupiter  is  five  times  as  far  as  the 
Earth  is  from  the  Sun.  The  intensity  of  the  solar  heat 
that  we  receive  here  has  therefore  to  be  reduced  in  the 
.proportion  of  25  to  1,  if  we  would  learn  what  the  intensity 
of  solar  heat  is  like  on  the  surface  of  Jupiter.  If  the 
intensity  of  sunbeams  were  reduced  to  the  twenty-fifth  part 
of  what  it  is  at  present,  it  could  hardly  be  an  adequate 
cause  for  the  production  of  vast  tempests  on  the  Earth. 
We  are  therefore  obliged  to  look  for  some  other  agent  than 
sun  heat  for  an  explanation  of  those  storms  by  which  the 
surface  of  Jupiter  is  so  frequently  distracted.  This  leads 
to  the  consideration  of  a  very  instructive  point  in  connexion 
with  the  physical  conditions  at  present  prevailing  on  Jupi- 
ter. It  seems  now  certain  that  although  the  planet  receives 
no  more  than  a  comparatively  small  share  of  sunbeams, 
yet  that  from  some  source  other  than  the  Sun  it  is  pro- 
vided with  a  supply  of  heat  abundantly  adequate  for  the 
generation  of  mighty  atmospheric  disturbances. 

§  133.  Internal  Heat.  All  evidence  points  to  the  fact 
that  the  internal  parts  of  the  great  planet  which  we  are 
now  considering  must  be  in  a  highly  heated  condition. 
It  is  indeed  probable  that  Jupiter  is  so  hot  that,  even  if 
there  was  a  solid  surface  beneath  that  cloud-laden  atmos- 
phere, water  could  not  rest  upon  it.  It  would  seem  that 
the  temperature  is  such  that  water  would  boil  away  from 
that  surface,  and  be  driven  off  into  vapour.  Let  us  imagine, 
for  the  sake  of  illustration,  that  this  Earth  of  ours  were  to 
become  so  hot  that  even  at  the  surface  it  was  about  the 
temperature  of  boiling  water,  and  was  doubtless  much 
hotter  still  in  the  interior.  Imagine  the  floor  at  the 


Jupiter  125 

bottom  of  the  sea  to  become  similarly  heated  and  to  be 
supplied  with  practically  unlimited  heat  from  beneath. 
Then  it  is  plain  that  all  the  water  in  every  river  and  every 
ocean  would  be  evaporated  and  turned  into  steam,  and 
ascending  into  the  atmosphere  would  form  a  stupendous 
mass  of  dense  and  impenetrable  clouds.  There  can 
hardly  be  a  doubt  that  something  of  this  kind  represents 
the  present  state  of  Jupiter.  The  constant  passage  of 
heat  from  the  interior  of  the  planet  to  its  surface  main- 
tains enormous  masses  of  material  in  the  form  of  clouds 
in  this  atmosphere,  while  local  inequalities  in  the  transfer 
of  this  heat  generate  such  disturbances  that  the  incessant 
storms  with  which  the  mighty  planet  is  distracted  can  be 
accounted  for. 

§  134.  The  Future  of  Jupiter.  It  seems  probable  that 
there  was  a  time  when  this  Earth  was  highly  heated  in 
the  manner  just  supposed.  There  was  a  time  when  water 
could  not  rest  upon  this  globe  in  a  liquid  form,  so  that  just 
as  the  present  condition  of  Jupiter  illustrates  an  earlier 
stage  in  the  Earth's  history,  so  the  present  state  of  the 
Earth  may  perhaps  foreshadow  the  remote  future  which 
awaits  the  greatest  planet  of  our  system,  when  in  the 
fulness  of  time  it  shall  also  have  parted  with  its  redun- 
dant heat  and  when  the  materials  of  its  future  ocean  at 
present  forming  its  mighty  clouds  of  vapour  shall  have 
l>een  collected  upon  its  surface. 

§  135.  Satellites.  The  astronomical  student  who 
lias  become  possessed  of  a  telescope  is  always  specially 
interested  in  the  beautiful  system  of  Moons  attending 
Jupiter.  He  is  delighted  at  their  incessant  changes,  he 
observes  them  night  after  night  as  they  move  from  one  side 
of  the  great  planet  to  the  other,  he  looks  out  for  their 
eclipses  in  which  the  light  is  cut  off  from  them,  because 
they  have  plunged  into  the  shadow  which  the  vast  globe 
casts  out  behind  it  into  space.  He  watches  for  their 


126 


Astronomy 


reappearance  at  the  expected  moment.  If  he  has  a  good 
telescope  he  will  occasionally  be  so  fortunate  as  to  observe 
the  satellite  in  the  act  of  transit  across  the  surface  of  the 
planet ;  he  will  note  the  delicate  beauty  of  the  shadow 
cast  by  the  satellite  and  he  will  follow  its  movement  over 
the  bright  surface.  He  will  also  sometimes  be  able  to 
witness  the  occupation  of  one  of  these  little  moons,  as  it 
retires  behind  the  mighty  globe,  in  due  course  reappear- 
ing on  the  other  side. 

Fig.  22  shews  Jupiter  and  the  orbits  of  his  four  satel- 
lites. The  planet  and  each  of  the  satellites  is  accompanied 
by  an  enormously  long  shadow.  The  innermost  satellite 
(I)  is  shewn  in  transit  over  the  disc  at  a,  its  long  shadow 
lying  clear  of  the  planet.  The  second  (II)  appears  from  the 


Fig.  22.    Transits,  Eclipses,  and  Occupations  of  Jupiter's  Satellites. 

earth  to  be  clear  of  the  planet,  but  it  is  casting  a  shadow 
upon  the  disc  at  b.  The  third  satellite  is  represented  as 
undergoing  an  eclipse  in  the  shadow  of  the  planet  at  c, 
while  IV,  though  clear  of  the  shadow,  is  still  invisible  from 
the  earth  being  occulted  by  the  planet,  as  represented  at  d. 
§  136.  The  Fifth  Satellite.  For  nearly  three  centu- 
ries the  satellites  of  Jupiter  have  received  the  unremit- 
ting attention  of  astronomers,  and  a  fresh  interest  has 
now  been  created  with  regard  to  them  in  consequence  of 


Jupiter  127 

the  remarkable  discovery  made  by  Professor  Barnard  in 
September,  1892.  The  four  well-known  satellites  are  all 
about  the  same  size,  and  are  all  about  the  same  apparent 
brightness.  A  telescope  which  can  shew  any  one  of  them 
is  generally  competent  to  shew  all  four  when  other  circum- 
stances are  suitable.  The  satellites  of  Jupiter  form  what 
are  called  'easy  objects'  in  the  astronomer's  vocabulary. 
For  as  already  mentioned,  the  very  smallest  telescopic 
power  will  suffice  to  shew  them.  Instances  have  indeed 
been  recorded  in  which  some  of  the  satellites  of  Jupiter 
have  been  actually  seen  with  the  unaided  eye.  No  doubt 
such  observations  have  been  very  exceptional,  and  it 
suffices  to  mention  that  no  one  ever  discovered  these  little 
objects  until  Galileo  turned  the  newly  made  telescope 
upon  them.  None  of  the  keen  eyes  of  antiquity  in  the 
days  before  telescopes  were  invented  ever  succeeded  in 
establishing  the  existence  of  these  little  objects. 

It  was  however  reserved  for  Professor  Barnard  to  aston- 
ish the  astronomical  world  with  the  announcement  that  the 
four  satellites  of  Jupiter  had  an  extremely  faint  companion 
in  their  wanderings.  This  little  body  is  indeed  so  small 
that  just  as  the  older  satellites  are  the  very  easiest  of 
telescopic  objects,  so  the  new  satellite  is  one  of  the  most 
difficult.  Only  two  or  three  of  the  most  potent  telescopes 
have  as  yet  sufficed  to  disclose  it,  only  two  or  three  of  the 
most  experienced  and  keen-eyed  astronomers  have  as  yet 
observed  it.  The  minuteness  of  the  object  will  explain 
the  difficulty  of  the  observation.  It  has  been  estimated 
that  if  Jupiter  were  himself  represented  by  a  cocoanut 
his  tiny  satellite  would  be  about  as  large  as  a  grain  of 
mustard  seed  on  the  same  scale.  It  is  a  matter  of  but  little 
wonder  that  the  satellite  has  hitherto  succeeded  in  eluding 
all  telescopes  save  those  of  the  most  powerful  description. 

§  137.  Size  and  Weight  of  Jupiter.  Much  yet  re- 
mains to  be  learned  about  this  vast  planet,  which  is  so 


128  Astronomy 

large  that  it  would  take  110  fewer  than  thirteen  hundred 
globes  each  as  large  as  the  Earth  all  rolled  into  one  to 
equal  it  in  size.  There  is,  however,  another  very  interesting 
line  of  reasoning  by  which  we  can  shew  how  the  apparent 
size  of  Jupiter  is  largely  due  to  the  fact  that  its  globe 
is  surrounded  by  mighty  masses  of  clouds.  Among  the 
great  problems  which  astronomers  can  solve  in  their  efforts 
to  measure  the  various  celestial  magnitudes,  not  the  least 
remarkable  is  the  determination  of  the  actual  masses  of  the 
planets.  By  the  help  of  its  satellites  we  have  been  able 
to  discover  the  actual  weight  of  this  great  planet  Jupiter. 
No  doubt  we  might  reasonably  expect  that  the  weight 
of  Jupiter  should  be  stupendous  and  so  it  assuredly  is. 
There  is  no  use  in  trying  to  express  it  in.  tons,  we  can 
give  a  notion  of  the  weight  of  Jupiter  much  more  usefully 
by  comparing  it  with  the  weight  of  our  own  Earth.  It  has 
been  found,  as  might  have  been  expected,  that  the  mighti- 
est of  the  planets  is  much  heavier  than  the  Earth.  Indeed 
it  weighs  about  three  hundred  times  as  much.  But  the  real 
wonder  seems  to  be  not  that  Jupiter  is  so  heavy,  but  that 
it  is  not  a  great  deal  heavier.  Estimated  by  volume  the 
planet  is  thirteen  hundred  times  as  big  as  the  Earth,  yet 
now  we  find  that  when  tested  by  weight  it  has  no  more 
than  three  hundred  times  the  mass  of  the  Earth.  In  these 
figures  we  see  at  once  evidence  of  a  great  contrast  between 
the  actual  structure  of  the  two  globes.  If  Jupiter  had 
been  built  of  the  same  materials  as  the  Earth,  and  if  those 
materials  were  in  the  same  physical  condition,  then  we 
should  expect  that  as  Jupiter  was  thirteen  hundred  times 
as  big,  so  it  should  also  be  thirteen  hundred  times  as  heavy. 
When  therefore  we  find  so  great  a  disparity  between  the 
weight  of  Jupiter  and  its  bigness  we  may  feel  assured  that 
there  must  be  some  very  radical  difference  between  its 
constitution  and  the  constitution  of  the  Earth.  The  clouds 
with  which  Jupiter  is  thickly  encompassed  will  of  course 


Jupiter  129 

suggest  a  method  of  accounting  for  the  discrepancy.  The 
fact  is  that  the  Jupiter  which  we  measure  is  enormously 
swollen  by  the  abundance  of  the  clouds  with  which  it  is 
enfolded.  It  is  these  clouds  which  give  to  Jupiter  a  bulk 
altogether  out  of  proportion  to  its  weight. 


CHAPTER  X 

SATURN 

So  far  as  mere  greatness  is  concerned  Saturn  has  to  take 
the  second  place  among  the  planets.  The  neighbouring 
planet  Jupiter  has  a  volume  which  is  twice  as  large.  Nor 
does  the  globe  of  Saturn  offer  much  attraction  to  an 
astronomer.  It  does  not  present  for  our  examination  a 
beautiful  system  of  continents  with  canals  and  Arctic 
snows,  like  those  which  are  displayed  on  Mars.  Indeed 
Saturn  exhibits  no  details  on  his  globe  except  a  few  bands 
which  are  but  ill-defined,  and  certain  marks  which,  though 
very  faint,  have  fortunately  been  sufficiently  recognisable 
to  be  followed  during  the  planet's  rotation.  We  give  in 
the  Frontispiece  a  reproduction  of  a  beautiful  drawing  of 
Saturn  by  Prof.  E.  E.  Barnard. 

§  138.  The  System  of  Rings.  The  feature  which 
makes  Saturn  peerless  among  all  the  bodies  in  the  Universe 
is  presented  in  the  exquisite  system  of  rings  by  which  he  is 
surrounded.  We  have  first  to  realise  the  dimensions  of  the 
objects  at  which  we  are  looking.  The  outermost  ring  is 
about  a  hundred  and  seventy  thousand  miles  in  diameter. 
How  great  this  really  is  will  be  understood  if  we  reflect  that 
the  magnitude  just  stated  is  more  than  twenty  times  the 
diameter  of  the  Earth.  The  thinness  of  the  rings  is  also  a 
130 


Saturn  131 

characteristic  which  merits  special  notice.  Under  certain 
circumstances  it  will  happen  that  the  ring  is  turned  edge- 
wise towards  the  observer.  In  this  case  it  requires  a 
powerful  telescope  to  render  it  visible. 

§  139.  The  Structure  of  the  Rings.  The  first  question 
which  is  suggested  in  the  study  of  the  ring  system  con- 
cerns its  relation  to  the  central  globe.  It  has  first  to  be 
noticed  that  the  rings  are  not  attached  to  the  globe  by  any 
physical  structure.  They  are  in  fact  poised  round  the 
globe  quite  freely  and  the  globe  stands  majestically  in 
the  centre  of  the  circle  which  forms  the  inner  margin  of 
the  inner  ring.  At  first  it  seems  difficult  to  imagine  by 
what  contrivance  the  ring  remains  continually  balanced  in 
the  same  position.  How  is  it  that  the  ring  and  the  globe 
remain  associated  together  during  their  revolutions  around 
the  Sun  without  ever  being  displaced  from  their  relative 
positions  ?  This  we  shall  consider  when  we  have  explained 
the  discovery  which  has  shewn  the  nature  of  the  rings. 
The  main  portion  of  the  ring  is  divided  into  an  outer  and 
an  inner  part  by  a  dark  line  which  bears  the  name  of 
Cassini's  Division.  This  line  is  known  to  be  an  open  space 
and  through  it  the  actual  globe  of  the  planet  has  been 
sometimes  seen.  The  outer  of  the  two  rings  thus  sepa- 
rated is  itself  divided  into  two  parts,  by  a  line  more  diffi- 
cult to  see  than  Cassini's  line.  Both  of  these  lines  may  be 
described  as  circles,  the  centre  of  each  being  at  the  centre 
of  Saturn.  This  outer  line  is  however  not  quite  dark,  so 
that  it  is  clear  it  cannot  be  regarded  as  an  absolute  line 
of  separation  between  two  portions  of  the  ring.  It  also 
appears  that  besides  these  lines  which  have  been  referred  to 
there  are  certain  other  faint  lines  which  become  discernible 
under  exceptionally  favourable  conditions. 

§  140.  "  The  Crape-Ring."  But  the  most  delicate  part 
of  the  Saturnian  system  is  clearly  the  so-called  "crape- 
ring."  This  exquisite  structure  extends  from  the  inner 


132  Astronomy 

edge  of  the  bright  ring  about  half-way  in  towards  the  globe 
of  the  planet.  It  is  called  the  "  crape-ring "  because  it 
possesses  the  senii-transparency  of  such  a  fabric  as  crape. 
The  proof  of  the  partial  transparency  of  the  crape-ring 
is  afforded  occasionally  when  the  edge  of  the  solid  globe 
of  the  planet  can  be  seen  right  through  this  part  of  the 
ring. 

§  141.  A  Simple  Model  of  the  Saturnian  System.  We 
may  conveniently  represent  the  rings  in  this  way.  Take  a 
little  globe,  three  inches  in  diameter,  to  represent  Saturn, 
then  on  a  thin  piece  of  card  make  two  concentric  circles, 
of  which  the  outer  has  a  diameter  of  seven  inches  and  the 
inner  a  diameter  of  six  inches.  Cut  out  this  ring,  it  will 
represent  the  outer  ring  of  the  planet.  The  inner  ring 
may  be  similarly  illustrated  by  two  circles  of  five  and 
three-quarter  and  four  and  one-quarter  inches  in  diameter 
respectively.  If  we  now  place  the  latter  of  these  rings  in 
the  centre  of  the  former  and  the  globe  representing  Saturn 
symmetrically  in  the  central  aperture,  we  shall  have  a 
model  of  the  principal  parts  of  the  Saturnian  system 
which  may  render  the  explanation  which  is  to  follow 
more  readily  intelligible.  The  crape-ring  is  interior  to 
the  two  rings  already  mentioned. 

§  142.  Stability  of  the  Rings.  Ever  since  the  days 
of  Newton  it  has  been  found  necessary  to  reconcile  the 
observed  facts  of  Astronomy  with  the  law  of  universal 
Gravitation.  It  was,  therefore,  essential  to  explain  how 
each  of  the  rings  of  Saturn  could  remain  thus  accurately 
telanced  with  the  globe  itself  in  the  centre.  At  a  first 
glance  nothing  might  have  seemed  easier.  If  the  globe 
of  Saturn  lay  exactly  in  the  middle  of  the  ring,  as  the 
planet  pulls  the  ring  equally  all  round  and  as  the  posi- 
tion is  one  of  perfect  symmetry  the  various  pulls  would 
neutralise  each  other,  and  thus  the  Saturnian  system  would 
be  an  enduring  one.  A  little  further  reflection,  however, 


133 

shews  that  this  simple  explanation  would  not  answer. 
Saturn  could  no  more  lie  balanced  perfectly  in  the  middle 
of  its  ring  than  an  egg  could  stand  balanced  on  one  end. 
The  position  is  in  both  cases  one  of  unstable  equilibrium. 
It  is  an  equilibrium  which  Nature  does  not  like.  It  is 
therefore  impossible  to  meet  the  claims  of  universal 
gravitation  by  saying  that  the  ring  of  Saturn  stands 
poised  symmetrically  round  the  central  globe. 

§  143.  Mechanical  Condition.  The  appearance  of  the 
Saturnian  rings  in  the  telescope  certainly  suggests  that 
they  are  formed  from  sheets  of  some  solid  material. 
But  a  few  considerations  can  easily  be  adduced  to  shew 
the  insuperable  mechanical  difficulties  which  attend  such  a 
supposition.  It  will  be  obvious  that  we  may  think  of  the 
ring  as  formed  of  two  semi-circular  arches  placed  together 
base  to  base.  We  may  consequently  apply  the  well-known 
mechanical  principle  which  governs  the  construction  of  the 
arch  to  throw  some  light  upon  the  question  as  to  whether 
the  rings  of  Saturn  can  be  composed  of  solid  material. 

§  144.  Pressure  of  an  Arch.  I  do  not  suppose  that 
in  any  arch  as  yet  constructed  by  man  with  the  build- 
ing materials  at  his  disposal  the  utmost  possible  span 
has  ever  yet  been  reached.  It  may  well  be  that,  with 
certain  kinds  of  stone,  arches  considerably  wider  than 
the  beautiful  Grosvenor  Bridge  which  spans  the  Dee  at 
Chester  could  be  constructed ;  it  is  however  quite  certain 
that  though  engineers  may  not  heretofore  have  had  occa- 
sion to  erect  the  largest  possible  arches,  yet  there  must  be 
a  well-defined  limit  which  no  arch  can  transcend.  The 
greater  the  span  the  greater  is  the  crushing  pressure  to 
which  the  stones  of  the  bridge  are  subjected.  The  stone 
has  of  course  but  a  limited  capacity  for  resistance  to 
pressure  and  this  consideration  determines  the  size  of 
the  largest  possible  arch. 

§  145.    The  Rings  are  not  Solid.     But  in  the  outer  ring 


134  Astronomy 

of  Saturn,  or  rather  in  one-half  of  that  ring,  we  have  an 
arch  of  gigantic  span  amounting  indeed  to  considerably 
more  than  one  hundred  thousand  miles.  If  this  arch 
were  indeed  formed  of  solid  material  the  elements  em- 
ployed in  its  construction  would  be  subjected  to  pressure 
thousands  of  times  greater  than  the  pressures  which  could 
possibly  be  withstood  by  any  materials  such  as  those  of 
which  the  Solar  System  is  constructed.  It  must  be  re- 
membered that  the  pressure  which  would  prove  fatal  to 
the  stability  of  the  terrestrial  arch,  if  that  arch  exceeded 
a  certain  span,  arises  from  the  weight  of  the  materials  of 
which  the  bridge  itself  is  built.  The  magnitude  of  the 
pressures  on  the  Saturnian  arch  would  be  such  that  even 
if  it  had  been  built  from  materials  a  thousand  times 
tougher  than  the  toughest  steel  a  collapse  would  be 
inevitable.  We  are  therefore  led  to  examine  how  the 
intensity  of  the  pressure  might  be  reduced,  and  there  is 
one  obvious  method  by  which  at  all  events  some  allevia- 
tion could  be  effected.  If  the  ring  were  endowed  with  a 
movement  of  rotation  round  the  centre  of  the  planet  the 
centrifugal  force  with  which  its  parts  tend  to  fly  outwards 
would  of  course,  to  a  certain  extent,  neutralise  the  pull 
exerted  inwards  by  the  attraction  of  the  planet.  If  indeed 
the  ring  were  very  narrow,  as  narrow  as  we  know  it  is  thin, 
it  is  conceivable  that  such  a  velocity  of  rotation  might  be 
communicated  as  would  just  counteract  the  effect  of  the 
attraction.  The  pressure  on  the  materials  would  thus  be 
reduced,  and  so  far  as  our  present  argument  is  concerned, 
there  would  be  no  difficulty  in  conceiving  how  a  ring,  not- 
withstanding its  many  thousand  miles  in  diameter,  might 
be  sustained  around  Saturn  in  the  centre.  But  of  course 
the  ring  of  Saturn  has  a  width  many  times  greater  than  we 
have  supposed.  If  that  ring  were  indeed  a  plate  of  solid 
material  it  would  require,  in  order  to  equilibrate  the  strain 
at  its  outer  margin,  to  revolve  in  one  particular  period,  and 


Saturn  135 

in  a  different  period  if  the  strain  were  to  be  abated  on  the 
inner  margin.  Even  if  the  ring  rotated  with  an  average 
velocity,  which  might  have  the  effect  of  neutralising  the 
pressure  in  the  middle  of  the  ring,  the  pressure  would  be 
still  great  enough  to  crush  the  inner  parts  while  the  outer 
parts  would  be  rent  asunder.  These  are,  however,  only 
some  of  the  difficulties  which  follow  on  the  supposition 
that  the  rings  of  Saturn  are  really  the  solid  objects  which 
they  seem  to  be. 

§  146.  Dynamical  Theory.  The  pen  of  the  mathe- 
matician has  here  proved  a  more  subtle  instrument  than 
the  telescope  of  the  astronomer  for  investigating  the 
texture  of  the  rings  of  Saturn  —  Roche  first  shewed,  what 
Maxwell  afterwards  proved  independently,  that  it  was 
impossible  for  either  the  outer  ring  or  the  inner  ring  to 
be  a  solid  object.  It  further  appeared  that  there  were 
conclusive  dynamical  arguments  against  the  belief  that  the 
rings  we  see  are  composed  of  a  myriad  of  thin  concentric 
rings  each  formed  of  solid  material.  The  analysis  also 
demonstrated  that  the  rings  could  not  be  composed  of  fluid 
materials.  It  was  finally  demonstrated  that  the  only 
conceivable  mechanical  condition  in  which  the  matter 
constituting  the  rings  of  Saturn  could  exist  would  be  in 
the  form  of  myriads  of  comparatively  small  independent 
objects,  each  revolving  like  a  moon  round  Saturn  in  an 
orbit  of  its  own  and  each  with  its  proper  periodic  time. 
In  order  to  account  for  the  apparent  continuity  which  the 
rings  present,  as  seen  in  the  telescope,  it  was  only  necessary 
to  suppose  that  these  little  bodies  were  sufficiently  nu- 
merous and  sufficiently  close  together  to  form  a  sort  of 
swarm  whose  collective  light  would  give  the  appearance 
of  a  continuous  object,  though  the  several  individuals  of 
the  swarm  could  not  be  separately  discerned.  The  precise 
size  of  these  objects  is  immaterial.  They  might  be  no 
larger  than  the  motes  in  the  sunbeam  or  than  the  grains 


136  Astronomy 

of  sand  on  the  seashore,  they  might  be  as  large  as  moun- 
tains or  even  larger  still ;  the  essential  point  is,  however, 
that  whatever  their  dimensions  may  be,  those  dimensions 
are  small  relatively  to  the  breadth  of  Saturn's  rings. 

§  147.  The  Rings  are  Composed  of  Small  Particles. 
It  thus  came  to  pass  that  astronomers  received  as  an 
accepted  doctrine  the  theory  that  the  rings  of  Saturn 
must  really  be  formed  of  small  particles,  moving  round 
the  planet  in  their  several  paths.  Nor  was  this  belief 
diminished  appreciably  by  the  reflection  that  visual  con- 
firmation of  the  fact  seemed  quite  beyond  all  possibility 
of  realisation. 

§  148.  Keeler's  Observations.  Great  then  was  the 
interest  excited  among  all  engaged  in  the  study  of  the 
heavens  by  the  announcement  of  certain  remarkable 
astronomical  observations  by  Professor  Keeler  with  the 
aid  of  the  spectroscope  upon  the  velocity  with  which 
different  parts  of  the  ring  revolve  around  the  planet.  He 
has  demonstrated  in  a  manner  alike  ingenious  and  convinc- 
ing that  the  rings  of  Saturn  have  as  a  matter  of  observed 
fact  that  precise  character  which  the  beautiful  dynamical 
theory  attributed  to  them.  Sir  William  Huggins  some 
time  ago  made  the  remarkable  statement  that  of  all  the 
discoveries  in  the  celestial  regions  which  the  spectroscope 
had  been  the  means  of  giving  us,  perhaps  the  most 
important  would  ultimately  be  found  to  be  the  revelation 
of  particular  movements  which  successfully  elude  all  other 
means  of  detection.  The  facts  already  brought  to  light 
have  certainly  corroborated  these  statements,  and  now  the 
astonishing  announcement  made  by  Professor  Keeler,  as 
to  what  he  has  found  in  the  rings  of  Saturn,  carry  the 
doctrine  a  very  long  way  towards  complete  demonstration. 

§  149.  The  Spectroscopic  Method  of  Observation. 
The  essential  feature  of  the  spectroscopic  method  of 
research  which  is  leading  to  such  a  great  extension  of 


Saturn  137 

our  knowledge  can  be  easily  described.  The  light  from 
a  celestial  object,  when  submitted  to  examination  by  the 
spectroscope,  presents  a  system  of  lines  characteristic  of 
the  peculiar  nature  of  the  light  which  that  object  dispenses. 
If,  however,  it  should  happen  that  the  object  is  travelling 
rapidly  towards  the  observer,  then  the  system  of  lines  is 
shifted  in  one  direction  while,  if  the  object  be  receding 
from  the  observer,  the  lines  are  shifted  in  the  opposite 
direction.  By  measuring  the  amount  through  which  these 
lines  are  shifted,  it  is  possible  to  determine  the  actual  speed 
at  which  the  body  is  moving  along  the  line  of  sight.  The 
significance  of  the  results  thus  obtained  arises  from  dif- 
ferent causes.  In  the  first  place,  the  movements  along 
the  line  of  sight  are  precisely  those  particular  movements 
which  entirely  elude  all  observations  made  in  the  ordinary 
manner.  The  fact  that  a  star  is  hurrying  towards  us,  or 
from  us,  does  not  produce  any  apparent  change  in  its  place 
on  the  heavens,  and  yet  it  is  only  by  alterations  in  the 
star's  place  that  movements  could  be  determined  by  those 
methods  which  alone  were  available  to  astronomers  before 
the  newer  method  came  into  use.  The  remoteness  of  the 
object  is  also  immaterial,  so  far  as  the  spectroscopic  method 
is  concerned.  The  process  is  equally  available  for  studying 
the  movement  of  a  planet  in  our  system,  or  of  a  star  which 
is  at  a  distance  millions  of  times  greater  than  are  any  of 
the  planets.  Nor  does  it  matter  either  how  small  may  be 
the  object  from  which  the  light  comes,  for  so  long  as  that 
light  is  perceptible  in  the  spectroscope,  the  dimensions  of 
its  source  do  not  concern  us.  Professor  Keeler  has  applied 
this  method  with  singular  practical  skill  to  the  study  of 
the  lings  of  Saturn.  He  has  thus  shewn  that  the  theory 
of  their  constitution  which  mathematicians  announced  so 
many  years  before  is  actually  borne  out  by  observation. 
The  sunlight  reflected  from  the  rings  of  Saturn  is  affected 
by  the  movements  of  the  little  particles  of  which  those 


138  Astronomy 

rings  are  composed  ;  these  movements  produce  such  altera- 
tions in  the  position  of  the  lines  of  the  spectrum  that  it 
has  been  found  possible  to  determine  the  pace  at  which  the 
different  parts  of  Saturn's  rings  are  moving.  It  has  thus 
been  rendered  absolutely  certain  that  the  rings  consist  of 
innumerable  myriads  of  small  particles,  each  pursuing  its 
own  course  as  an  independent  little  moon. 

Astronomers  look  with  much  admiration  upon  this  re- 
markable confirmation  of  a  purely  theoretical  deduction. 

§  150.  Satellites.  The  strange  and  beautiful  system 
of  rings  which  surround  this  planet  has  occupied  us  so  long 
that  we  have  but  little  space  left  to  devote  to  the  remark- 
able retinue  of  satellites  by  which  it  is  also  attended.  In 
this  respect,  however,  it  is  by  far  the  richest  of  all  the 
planets.  The  largest  of  these  satellites,  Titan,  was  dis- 
covered by  Huyghens  as  early  as  1655,  and  by  the  end 
of  the  seventeenth  century  four  more,  lapetus,  Khea,  Dione 
and  Tethys,  had  been  discovered.  Sir  William  Herschel 
added  two,  Mimas  and  Enceladus,  to  the  list.  In  1848 
Bond  in  America,  and  Lassell  in  England,  independently 
discovered  an  eighth  which  was  called  Hyperion,  and  so 
the  list  remained  for  half  a  century.  Last  year,  however, 
a  ninth  was  discovered  by  Prof.  W.  H.  Pickering  by  com- 
paring two  photographs  of  Saturn  and  his  surroundings 
which  had  been  taken  at  Arequipa  in  Peru  on  August  16th 
and  18th  respectively. 

The  following  is  a  list  of  these  bodies  Avith  the  dates 
of  their  discovery  and  the  times  in  which  they  perform 
their  revolutions  around  the  primary.  They  all  move  in 
very  nearly  circular  orbits  which  lie  as  nearly  as  possible 
in  the  plane  of  the  rings,  with  the  exception  of  the  orbit 
of  lapetus  which  is  inclined  at  an  angle  of  about  10°  to 
that  plane,  and  possibly  that  of  the  ninth  whose  motion  has 
not  yet  been  fully  determined. 


Saturn  139 


THE  SATELLITES  OF  SATURN 


Discovered  Periodic  Time 

<lavs        hrs.       min* 


Mimas 

1789 

6 

22 

36 

Enceladus 

1789 

1 

8 

53 

Tethys 

1684 

1 

21 

18 

Dione 

1684 

2 

17 

41 

Rhea 

1672 

4 

12 

25 

Titan 

1655 

13 

22 

41 

Hyperion 

1848 

21 

6 

35 

lapetus 

1671 

79 

7 

47 

Pickering's  Satellite 

1899 

400? 

CHAPTER   XI 
URANUS  AND  NEPTUNE 

§  151.  The  Discovery  of  Uranus.  The  five  older 
planets  have  been  known  since  prehistoric  times.  There 
was,  in  fact,  no  record  whatever  of  the  discovery  of  any  of 
the  planets  when  on  March  13th,  1781,  William  Herschel, 
who  then  occupied  the  position  of  Organist  at  the  Octagon 
Chapel  at  Bath,  made  one  of  the  greatest  discoveries  in  the 
annals  of  Science.  He  had  constructed  with  his  own  hands 
a  reflecting  telescope.  This  instrument  possessed  consider- 
able optical  perfection,  so  much  so  that  when  on  the  night 
in  question  he  was  examining  with  its  help  the  stars  in  the 
constellation  Gemini,  and  using  considerable  magnifying 
power,  his  attention  was  arrested  by  a  point  which  did 
not  present  the  appearance  of  an  ordinary  star.  It  had 
a  diameter  of  appreciable  dimensions.  The  perception  of 
this  fact  shewed  at  once  Herschel's  skill  as  an  observer,  as 
well  as  the  perfection  of  his  home-made  instrument.  This 
will  be  evident  when  what  was  subsequently  ascertained 
is  borne  in  mind.  It  appeared  that  previous  astronomers 
had  on  no  fewer  than  seventeen  occasions  observed  this 
object,  but  having  noticed  no  difference  between  it  and  an 
ordinary  star  had  paid  it  no  further  attention.  The 
penetrating  glance  of  Herschel  at  once  perceived  a  dif- 
140 


Uranus  and  Neptune  141 

fereuce.  He  looked  at  this  object  again  and  again;  he 
found  that  unlike  a  star,  as  to  its  appearance,  it  was  also 
unlike  in  the  fact  that  it  was  in  motion,  and  ere  long  it 
was  discovered  that  this  was  a  splendid  planet  revolving 
outside  the  orbit  of  Saturn.  This  new  object  like  the 
older  planets  revolved  in  a  nearly  circular  orbit  around 
the  Sun  and  moved  in  a  plane  but  little  inclined  to  the 
plane  of  the  ecliptic.  Subsequent  observation  shewed  that 
it  was  attended  by  four  satellites,  very  small  objects  only 
to  be  discerned  with  much  difficulty.  The  planet  received 
the  name  of  Uranus.  Its  diameter  is  four  times  as  great 
as  that  of  the  Earth  and  it  weighs  about  fifteen  times  as 
much. 

§  152.  Earlier  Observations.  On  search  being  made 
through  earlier  star  catalogues  it  was  found  that  Ura- 
nus had  been  observed  in  1690,  in  1712,  in  1715,  and  in 
1756.  On  all  these  occasions  the  planet  had  been  re- 
garded as  a  star  and  its  place  had  been  duly  set  down. 
When,  however,  Uranus  became  known  by  Herschel's 
discovery  it  was  possible  to  calculate  the  position  which 
it  held  in  the  heavens  on  these  earlier  dates,  and  thus 
the  identity  of  the  planet  with  the  supposed  stars  was 
established.  If  confirmation  were  wanted  it  was  found 
in  the  fact  that  on  subsequent  comparison  of  the  star 
catalogues  with  the  heavens  it  was  found  that  no  stars 
were  visible  in  the  places  referred  to.  The  planet  had 
been  there  seen  and  its  place  had  been  set  down,  but 
of  course  the  planet  then  moved  away  and  left  its  place 
vacant. 

§  153.  Discrepancies  in  the  Motion  of  Uranus.  These 
early  observations  of  Uranus  proved  however  to  be  of 
the  greatest  importance,  inasmuch  as  they  enabled  the 
track  which  Uranus  follows  through  the  heavens  to  be 
determined  with  much  accuracy.  For  each  revolution 
Uranus  requires  no  less  than  84  years,  and  consequently 


142  Astronomy 

it  would  be  impossible  to  determine  its  orbit  with  any 
very  high  degree  of  precision  by  a  few  observations 
separated  by  short  intervals.  When,  however,  the  planet 
had  been  under  observation  for  some  time  its  orbit  was 
accurately  determined.  It  was  also  possible  to  determine 
the  orbit  of  Uranus  by  means  of  the  early  observations,  in 
which  the  planet  had  been  mistaken  for  a  star.  And  when 
the  two  orbits,  as  deduced  from  these  two  sets  of  observa- 
tions, were  compared  together,  it  might  have  been  expected 
that  the  tracks  they  indicated  for  the  moving  body  would 
be  identical.  But  this  anticipation  was  not  realised.  The 
orbit  of  Uranus  as  indicated  by  later  observations  was  not 
the  same  as  that  indicated  by  the  earlier  observations. 

This  disagreement  excited  much  interest  among  as- 
tronomers. Bouvard  published  in  1821  an  account  of 
the  movement  of  Uranus  in  which  he  dwelt  upon  the 
discrepancies,  and  he  shewed  that  they  could  only  be 
accounted  for  by  supposing  that  they  were  due  to  some 
extraneous  and  unknown  influence  which  disturbed  the 
movements  of  the  planet.  Of  course  the  known  planets, 
Jupiter  and  Saturn,  exercised  their  disturbing  effect  upon 
Uranus  also,  but  these  could  be  allowed  for,  and  when 
this  was  done  it  was  found  that  there  were  discrepancies 
still  outstanding.  He  even  went  so  far  as  to  suggest 
that  this  extraneous  influence  was  perhaps  due  to  the 
attraction  of  some  unknown  planet  which  circulated  in  an 
orbit  exterior  to  that  of  Uranus. 

A  magnificent  mathematical  problem  was  thus  sug- 
gested. It  was  no  less  than  the  investigation  of  the  orbit 
of  this  unknown  planet  from  the  effect  which  its  attrac- 
tion produced  on  the  movements  of  Uranus. 

§  154.  The  Discovery  of  Neptune.  If  there  were 
indeed  an  outer  planet,  certain  facts  with  reference  to  it 
might  at  all  events  be  predicted  from  the  analogy  of  the 
planets  already  known.  It  might  fairly  be  assumed  that 


Ui'unus  and  Neptune  143 

this  outer  planet  revolved  around  the  Sun.  in  a  nearly 
circular  orbit,  and  that  the  plane  in  which  it  moved  was 
practically  coincident  with  the  plane  in  which  all  the 
other  great  planets  revolved.  It  was  also  possible  to  take 
a  still  further  step  by  a  reasonable  conjecture  as  to  what 
the  distance  of  the  new  planet  from  the  Sun  might  be 
expected  to  be.  There  exists  indeed  a  very  curious  rela- 
tion which  connects  together,  in  an  approximate  fashion, 
the  distances  of  the  important  planets.  This  relation  is 
generally  known  by  the  name  of  Bode's  law.  We  must 
be  careful  to  distinguish  it  from  such  laws  as  those  of 
Kepler ;  the  latter  are  founded  on  the  laws  of  gravitation, 
and  thus  rest  on  a  mathematical  basis,  while  Bode's  law 
must  be  described,  at  least  so  far  as  our  present  know- 
ledge extends,  as  purely  empirical.  It  appears  to  be  true, 
but  mathematicians  have  not  yet  been  able  to  assign  any 
reason  why  it  should  be  so.  In  any  case  we  have  to  note 
that  its  truth  is  only  of  an  approximate  character. 

It  is  however  worthy  of  enunciation.     We  write  the 
following  series  of  numbers : 

0,  3,  6,  12,  24,  48,  96. 

It  is  easy  to  remember  this  series  by  the  circumstance 
that  after  the  two  first  figures  have  been  written  down 
each  number  is  double  the  one  which  precedes  it.  Let  us 
now  alter  this  series  by  adding  four  to  each  of  these 
numbers ;  the  TOAV  of  figures  then  becomes : 

4,  7,  10,  16,  28,  52,  100. 

This  series  of  figures  bears  a  remarkable  relation  to  the 
Solar  System.  With  the  exception  of  the  fifth  figure,  28, 
the  numbers  we  have  set  down  are  approximately  propor- 
tional to  the  distances  of  the  several  principal  planets 
from  the  Sun.  We  may  regard  the  number  28  as  repre- 
senting collectively  the  positions  of  the  asteroids.  In 


144  Astronomy 

fact,  if  we  denote  the  distance  of  the  Earth  from  the  Sun 
by  the  number  10,  we  may  represent  the  distances  of  the 
various  bodies  of  the  Solar  System  as  follows : 

Mercury    Venus      Earth        Mars    Asteroids    Jupiter      Saturn 

3-9        7-2        10        15-2        28         52-9       954 

These  numbers  agree  moderately  well  with  those  indicated 
by  Bode's  law.  It  was  further  found  when  Uranus  was 
discovered  that  its  distance  from  the  Sun,  as  represented 
by  the  law,  would  be  196,  which  does  not  differ  much  from 
191-8,  which  is  found  by  multiplying  the  actual  distance 
of  Uranus,  expressed  in  terms  of  the  mean  radius  of  the 
Earth's  orbit,  by  10.  Thus  the  law  of  Bode  gave  a  sort  of 
suggestion  as  to  what  the  distance  of  the  planet  should  be 
if  there  were  such  a  planet  revolving  beyond  Uranus.  On 
the  same  scale  as  those  already  adopted  the  law  would 
indicate  388  as  the  distance  of  the  planet  immediately 
outside  Uranus,  and  this  served  in  some  degree  as  a  guide 
to  the  unknown  body.  It  should  however  be  mentioned 
that  as  the  results  actually  turned  out  the  law  of  Bode 
was  in  this  case  considerably  astray.  The  actual  distance 
of  Neptune  was  discovered  to  be  no  more  than  3004. 

The  search  for  this  unknown  object  was  undertaken 
independently  by  two  mathematicians,  by  Le  Verrier  in 
France  and  by  Adams  in  England.  It  was  shewn  by 
Le  Verrier  that  these  discrepancies  could  be  completely 
reconciled  by  the  existence  of  an  outer  planet,  of  which  he 
determined  the  orbit.  He  was  indeed  enabled  to  predict 
the  place  of  the  planet  so  accurately  that  on  the  memorable 
night  of  September  23, 1846,  this  new  planet  was  actually 
discovered  by  Dr.  Galle,  at  Berlin,  on  making  a  search  in 
the  very  spot  to  which  the  indications  of  Le  Verrier  guided 
him.  A.  search  had  however  been  commenced  previously 
by  Professor  Challis  at  Cambridge  in  accordance  Avith  the 
calculations  of  Adams,  and  Challis  had  actually  observed 


(',•«  n  it  f!    Hii«l     \'r ^tiili?  145 

the  planet  with  the  Northumberland  Equatorial  on  August 
the  4th  and  12th,  1846.  There  can  be  little  doubt  that 
from  Challis's  observations,  whenever  they  came  to  be  dis- 
cussed, the  planetary  nature  of  the  object  would  have  been 
fully  recognised.  Unfortunately  this  discussion  was  not 
undertaken  until  after  Dr.  Galle's  observations  had  been 
announced,  and  so  the  actual  priority  of  the  discovery  was 
lost  to  Adams.  The  scientific  world  has  long  since  agreed 
that  the  credit  for  this  brilliant  discovery  must  be  equally 
shared  between  Le  Verrier  and  Adams. 

The  planet  Neptune  brought  to  light  in  this  astonishing 
manner  is  not  visible  to  the  unaided  eye.  It  will,  generally 
speaking,  be  ranked  as  a  star  of  the  8th  magnitude.  Under 
a  high  magnifying  power  the  dimensions  of  the  planet 
l>eeonie  more  considerable  and  its  circular  disc  can  be 
perceived.  The  actual  diameter  of  Neptune  is  about  four 
times  the  diameter  of  the  Earth.  It  is  accompanied  by  a 
single  satellite.  This  planet  is,  so  far  as  we  know,  the 
outermost  planet  of  the  solar  system.  Nor  is  there  any 
reason  for  thinking  that  there  are  any  planets  beyond  it. 


CHAPTER   XII 

COMETS 

§  155.  Appearance  of  Comets.  Besides  the  planets  and 
their  satellites  there  are  other  bodies  of  a  very  different 
character  which  belong  also  to  the  Solar  System.  These 
bodies  are  Comets  and  Shooting  Stars.  A  comet  possesses 
generally  a  nucleus  of  more  or  less  brilliance,  surrounded 
with  a  vast  quantity  of  nebulous  material,  which  is  often 
extended  in  one  direction  so  as  to  form  a  tail.  Very 
frequently,  however,  comets  are  not  provided  with  this 
appendage,  and  sometimes  also  the  nucleus  is  very  faint, 
or  is  entirely  wanting.  Comets  vary  greatly  as  to  bright- 
ness; in  the  majority  of  cases  these  objects  are  merely 
telescopic,  but  sometimes  they  are  brilliant  and  most 
striking  phenomena.  Tor  instance,  the  comet  of  1680  is 
said  to  have  had  a  tail  so  great  that  it  stretched  across 
the  sky  through  an  arc  equal  to  90°,  and  others  have  been 
recorded  with  tails  even  still  greater. 

A  comet  is  generally  visible  for  only  a  brief  period. 
It  will  first  appear  suddenly  in  some  region  where  there 
has  been  nothing  previously  to  attract  attention,  and  then 
from  day  to  day  it  increases  in  brightness  and  varies  in 
shape;  sometimes  it  remains  visible  for  a  few  weeks, 
sometimes  for  months ;  occasionally  it  passes  so  close  to 
the  Sun  that  it  becomes  invisible  for  a  while,  and  then 
again  it  will  be  restored  to  view  on  passing  to  the  other  side 
14G 


Comets  147 

of  the  Sun,  and  gradually  receding  it  becomes  smaller  and 
ultimately  disappears. 

§  156.  Movement  of  Comets.  We  owe  the  explana- 
tion of  the  movements  of  the  comets  to  Newton.  He 
.saw  that,  as  a  consequence  of  the  law  of  gravitation, 
each  object  submitted  to  the  attraction  of  the  Sun  must 
revolve  in  a  conic  section  around  the  Sun.  The  conic 
sections  in  which  the  several  planets  move  are  ellipses, 
and  in  like  manner  many  of  the  comets  revolve  also  in 
ellipses.  But  the  shape  of  the  ellipse  is,  generally  speaking, 
very  different  in  the  two  cases.  All  the  important  planets 
have  elliptic  orbits  which  differ  but  little  from  the  circular 
form.  They  do  not  move  in  orbits  which  are  highly 
eccentric,  but  orbits  of  this  kind  are  found  to  prevail 
amongst  the  comets.  The  elliptic  tracks  in  which  comets 
revolve  are  generally  of  a  high  degree  of  eccentricity. 

Newton's  principle  however  shewed  that  any  conic 
section  was  a  possible  form  of  orbit  under  the  influence 
of  a  centre  of  attractive  force  such  as  that  exerted  by 
the  Sun.  Now  there  are  three  forms  of  conic  sections,  the 
ellipse,  the  parabola,  and  the  hyperbola.  By  the  movements 
of  the  planets  we  were  provided  with  excellent  illustrations 
of  revolution  in  elliptic  orbits  ;  the  comets  give  us  illustra- 
tions of  movements  in  the  two  other  types  of  curve.  The 
great  majority  of  comets  move  in  what  is  known  as  the 
parabola.  There  is  a  fundamental  difference  between 
movement  in  an  ellipse  and  movement  in  a  parabola, 
inasmuch  as  the  ellipse  is  a  closed  curve,  while  the 
parabola  is  not.  It  therefore  follows  that  if  a  comet  be 
moving  in  an  elliptic  track  it  will  return  to  its  original 
position  after  the  lapse  of  a  number  of  years,  greater  or 
less,  according  as  the  dimensions  of  that  track  are  greater 
or  less.  In  such  a  case  the  comet  will  be  periodic.  We 
may  expect  it  to  appear  over  and  over  again,  and  some 
very  striking  comets  are  of  this  character. 


148 

The  most  famous  example  of  this  class  is  the  comet 
of  Halley,  which  revolves  in  about  seventy-five  years.  It 
was  last  seen  in  1835  and  will  be  therefore  due  again  about 
1910.  The  movement  of  a  body  along  a  parabolic  track 
is  of  a  very  different  character.  The  comet  will  advance 
towards  the  Sun,  it  will  sweep  round  the  Sun  and  then  it 
will  commence  to  retreat.  As,  however,  the  parabola  is  not 
a  closed  curve,  it  follows  that  the  object  thus  started  will 
never  again  return.  Most  of  the  famous  comets,  of  which 
the  history  is  recorded,  appear  to  have  been  of  this  parabolic 
class ;  they  make  one,  and  only  one,  apparition.  The  most 
notable  comet  in  the  memory  of  those  still  living  appeared 
in  1858,  during  the  autumn  of  which  year  it  attracted 
universal  attention.  This  was  a  comet  of  the  kind  to 
which  I  have  referred,  which  have  only  a  single  recorded 
appearance. 

§  157.  Photographs  of  Comets.  The  photographic  pro- 
cesses which  are  now  so  useful  in  astronomy  have  been 
of  special  advantage  in  the  endeavour  to  represent  the 
comets.  As  there  is  no  phenomenon  ever  witnessed  in  the 
heavens  more  striking  than  a  great  comet,  so  there  is 
certainly  none  which  is  apparently  more  suited  for  that 
particular  kind  of  study  which  the  camera  permits.  The 
growth  of  the  photographic  art  has  been,  however,  so  recent 
that  up  to  the  present  no  comet  which  could  be  described 
as  really  splendid  has  presented  itself  for  examination  by 
our  plates.  Astronomers  are  still  anxiously  awaiting  the 
display  of  another  cometary  spectacle  similar  to  that 
which  burst  forth  in  1858. 

§  158.  Swift's  Comet,  1892.  Some  of  the  most  re- 
markable portraits  of  comets  which  have  yet  been  ob- 
tained are  due  to  Professor  Barnard  when  he  was  at  the 
Lick  Observatory.  We  shew  here  a  picture  of  Swift's 
Comet  taken  by  Barnard  in  the  year  1892.  It  may  be 
regarded  as  exhibiting  the  structure  typical  of  such 


SWIFT'S    COMET.    1892 

Photographed  by  PROF.  BARNARD,  April  19,  1892 

To  face  page  148 


Comets  149 

bodies  generally.  There  is  first  of  all  the  more  or  less 
circular  head,  and  then  there  is  the  tail  which  extends 
to  a  very  great  distance.  Whatever  may  be  thought  of 
the  dimensions  which  the  tail  appears  to  display  as  shewn 
on  the  plate,  there  can  be  no  doubt  that  the  actual  length 
of  this  important  feature  extended  to  many  millions  of 
miles.  It  had  long  been  known  that  the  greater  part  of 
a  comet  is  composed  of  materials  of  the  flimsiest  descrip- 
tion. There  appears  to  be  nothing  that  is  actually  solid 
in  any  part  of  the  body's  mighty  extent,  and  the  tail  con- 
sists of  material  which  is  specially  rarefied  and  diffused. 
This  is  abundantly  illustrated  by  the  circumstance  that 
very  faint  stars  can  be  seen  right  through  the  thickness 
of  the  tail  of  the  comet.  In  looking  at  such  stars  the  line 
of  vision  actually  pierces  through  a  volume  of  cometary 
material  hundreds  of  thousands  of  miles  in  thickness. 
Judging  from  its  effects  on  the  star,  the  comet's  tail  does 
not  seem  to  possess  as  much  opaque  material  as  is  in  a 
light  cloud  floating  on  a  summer  sky.  Faint  stars  would 
be  completely  extinguished  by  such  a  cloud,  and  yet  we 
often  see  stars  notwithstanding  the  interposition  of  a 
stupendous  thickness  of  comet.  This  fact  sufficiently 
demonstrates  the  extraordinary  rarity  of  the  materials  of 
which  a  comet  is  composed. 

An  interesting  feature  connected  with  the  comet  is 
brought  out  very  strikingly  by  the  photograph.  For  such 
pictures  long  exposures  are  indispensable.  The  objects  are 
sometimes  so  delicate  that  a  detailed  picture  cannot  be 
obtained  in  less  than  an  hour.  But  we  must  remember  that 
in  the  course  of  an  hour  the  apparent  diurnal  motion  of 
the  heavens  carries  both  the  stars  and  the  comets  through  a 
considerable  distance.  If  therefore  the  telescope  were  kept 
fixed  during  the  time  of  exposure  intelligible  pictures  of 
the  celestial  bodies  would  be  impossible.  It  is  accordingly 
necessary  to  guide  the  telescope  so  that  we  shall  follow  the 


150  Astronomy 

comet  in  such  a  way  as  to  enable  the  image  of  the  body 
always  to  be  impressed  on  the  same  part  of  the  plate. 

This  is  a  delicate  operation,  and  it  requires  much  care 
on  the  part  of  the  astronomer,  who  is  directing  the 
instrument,  while  the  exposure  is  in  progress.  But  in 
this  particular  application  of  celestial  photography  a  very 
interesting  point  must  be  observed.  The  comet  is  all 
the  time  pursuing  its  own  orbit  and  consequently,  while 
the  exposure  has  lasted,  the  comet  experiences  a  displace- 
ment relative  to  the  surrounding  stars.  As,  however,  it  is 
the  comet  which  it  is  desired  to  represent  the  telescope  has 
been  always  directed  to  that  object.  It  follows  that  the 
images  of  the  stars  which  would  of  course  have  been 
sharply  marked  points  if  the  telescope  had  been  guided  by 
their  movements,  were  transformed  into  little  streaks. 
These  streaks  may  be  regarded  as  exhibiting  both  in 
direction  and  in  magnitude  the  distance  through  which 
the  comet  had  moved  during  the  time  of  the  exposure. 

§  159.  Direction  of  Comets'  Tails.  A  further  instruc- 
tive point  is  brought  out  by  this  picture.  It  will  be 
noted  that  the  tail  of  the  comet  does  not  appear  to 
stream  out  along  the  direction  which  is  defined  by  the 
star  streak.  The  tail  of  the  comet  does  not  lie  along  its 
track  in  the  same  way  as  the  sparks  from  a  sky-rocket 
lie  along  the  track  which  the  sky-rocket  has  pursued.  The 
tail  of  the  comet  has  assumed  a  position  which  is  imposed 
by  the  direction  in  which  the  Sun  lies  relatively  to  it. 
Indeed,  it  is  a  general  rule  that  the  tail  of  a  comet  points 
away  from  the  Sun,  and  that  it  does  so  independently  of 
the  direction  in  which  the  comet  may  at  the  time  happen 
to  be  moving. 

§  160.  Discovery  of  Comets  by  Photography.  The 
camera  has  also  been  the  means  of  discovering  at  least 
one  comet  which  had  not  been  previously  detected  by 
the  ordinary  method  followed  in  searching  for  such  ob- 


Comet*  151 

jects.  Professor  Barnard  when  taking  a  photograph  of 
the  stars  in  the  ordinary  way  was  as  usual  guiding  the 
telescope  by  a  star  on  the  picture.  When  the  plate 
was  developed  the  stars  appeared  to  be  points  of  light, 
as  of  course  they  should.  There  was  however  one  object 
on  the  plate  which  did  not  present  the  appearance  of  a 
point,  it  rather  appeared  as  an  ill-defined  streak.  This 
was  clearly  an  indication  of  something  which  had  moved 
relatively  to  the  stars  during  the  time  of  the  exposure. 
A  little  further  examination  shewed  that  this  was  indeed 
a  new  comet,  whose  existence  was  thus  betrayed  by  the 
fact  of  its  displacement. 

Yet  one  more  achievement  of  the  camera  in  the  study 
of  the  comets  may  be  mentioned.  It  has  opened  up  a 
field  of  possible  developments  in  the  future.  It  has  been 
frequently  found  that  the  photographic  plate  will  repre- 
sent objects  which  cannot  be  seen  by  the  human  eye.  A 
comet  appeared  with  the  tail  about  2°  in  length  so  far  as 
mere  telescopic  examination  would  display  it.  It  is  how- 
ever certain  that  the  tail  of  this  comet  was  a  very  much 
larger  structure  than  the  mere  telescopic  picture  would 
lead  us  to  suppose,  for  the  photographs  display  a  tail  not 
less  than  five  times  as  long. 


CHAPTER  XIII 
SHOOTING  STARS 

§  161.  How  a  Shooting  Star  becomes  Visible.  An 
ordinary  shooting  star  is  really  a  very  small  object. 
Probably  the  shooting  stars  in  a  shower  are  bodies  com- 
parable in  size  with  the  pebbles  on  a  gravel  walk.  The 
shooting  star  is  rendered  visible  to  us  only  by  the  illumi- 
nation generated  by  the  heat  attending  its  plunge  into 
the  atmosphere  with  which  the  Earth  is  surrounded.  In 
open  space  the  little  object  which  is  to  be  presently  trans- 
formed into  a  streak  of  splendour  is  rushing  along  with 
a  speed  one  hundred  times  as  great  as  that  with  which 
the  swiftest  rifle  bullet  is  animated,  for  in  the  emptiness 
of  space  there  is  no  resistance  to  motion.  Directly  the 
missile  finds  itself  in  contact  with  the  atmosphere  it  ex- 
periences tremendous  obstruction.  As  it  rushes  through 
the  air  it  becomes  warmed  by  friction ;  the  friction  is 
indeed  so  great  that  the  object  becomes  red  hot  and 
white  hot,  until  at  last  it  is  actually  melted  and  trans- 
formed into  a  streak  of  brilliant  vapour.  Thus  it  is  that 
Ave  see  what  is  called  a  shooting  star. 

On  almost  any  night  that  is  clear  the  careful  observer 
will  see  several  such  objects.  The  majority  of  these  are 
such  exceedingly  small  bodies  as  to  be  dissipated  during 
152 


153 

their  flight  through  the  atmosphere.  But  from  time  to 
time  some  of  larger  size  are  observed,  and  in  some  cases 
even  they  have  partially  survived  the  fiery  ordeal  which 
the  atmospheric  resistance  presents  and  a  portion  of  their 
solid  mass  has  fallen  on  the  surface  of  the  Earth.  These 
objects  are  then  called  meteorites. 

§  162.  Showers  of  'Shooting  Stars.  It  occasionally 
happens  that  shooting  stars  appear  in  vast  multitudes, 
forming  what  is  known  as  a  shooting  star  shower.  The 
meteors  of  a  shooting  star  shower  are  found  in  each  case 
to  diverge  or  radiate  from  a  particular  point,  or  from  a 
small  area  of  the  sky,  and  in  general  the  appearance 
of  each  shower  is  confined  to  a  particular  time  of  year. 
Thus  there  are  the  shooting  stars,  to  mention  only  the 
most  important,  radiating  from  a  point  in  the  constella- 
tion Perseus  which  are,  from  this  circumstance,  called 
Perseids.  These  are  met  with  every  year  in  the  second 
week  of  August.  There  are  also  the  Andrornedids,  whose 
radiant  point  is  situated  in  the  constellation  Andromeda 
and  which  are  only  seen  about  November  27.  But  the 
most  interesting  of  all  the  periodical  shooting  star  showers 
are  the  Leonids,  whose  radiant  point  is  in  the  constellation 
Leo  and  which  are  met  with  about  the  middle  of  November. 
There  are  some  disturbing  causes,  which  we  need  not  at 
present  consider,  tending  to  bring  about  the  reappearance 
of  this  shower  at  a  later  and  later  date  at  each  recurrence, 
but  at  present  it  is  to  be  looked  out  for  between  November 
14th  and  16th.  Those  who  were  fortunate  enough  to  have 
witnessed  the  superb  display  of  shooting  stars  on  the  13th 
November,  1866,  will  probably  agree  that  it  was  the  most 
impressive  spectacle  they  ever  beheld  in  the  heavens. 

§  163.  The  Periodicity  of  the  Leonids.  The  recurrence 
of  this  shooting  star  shower  is  remarkable.  They  make 
a  special  appearance  on  an  average  every  thirty-three 
years  and  a  quarter.  It  may  well  be  asked  how  we  can 


l.">4  Astronomy 

venture  to  predict  a  shooting  star  shower  Avith  any  reason- 
able expectation  of  success.  It  may  at  once  be  admitted 
that  we  cannot  attempt  to  foretell  the  occurrence  of  shoot- 
ing star  showers  with  the  same  feeling  of  absolute  certainty 
as  we  have  in  the  prediction  of  an  eclipse  of  the  Sim  or 
the  Moon.  The  latter  depend  only  on  the  relative  posi- 
tions of  the  Sun,  the  Moon,  and  the  Earth,  and  thus 
involve  only  considerations  which  are  in  all  respects 
known  to  us.  In  the  case  of  the  movements  of  the 
shooting  stars  the  conditions  are  by  no  means  so  definite 
as  they  are  in  the  case  of  an  eclipse. 

Each  year  when  the  critical  date  in  November  comes 
round,  astronomers  are  accustomed  to  expect  rather  more 
shooting  stars  than  are  generally  to  be  seen  on  other  nights 
of  the  year.  But  the  display  of  these  Leonids  has  by  no 
means  the  same  significance  every  year.  Sometimes  a 
November  will  pass  in  which  but  few  meteors  will  be 
noticed  specially  belonging  to  this  group,  and  in  most  other 
years  the  Leonids  observed  by  astronomers  do  not  form 
any  spectacle  sufficient  to  command  the  particular  attention 
of  the  public.  But  it  has  sometimes  happened  that  the 
arrival  of  the  middle  of  November  has  been  marked  by  a 
display  of  countless  thousands  of  shooting  stars,  exhibit- 
ing a  spectacle  only  to  be  described  as  sublime. 

§  164.  Former  Showers.  For  nearly  a  thousand  years 
such  phenomena  have  from  time  to  time  been  noticed. 
Early  chroniclers,  who  had  not  the  faintest  idea  of  the  true 
character  of  the  apparition  which  to  their  astonishment 
suddenly  burst  forth,  set  down  accounts  of  what  they 
saw.  No  doubt  these  accounts  of  celestial  portents  have 
but  little  pretension  to  scientific  accuracy,  but  they  give 
us  at  least  the  dates  which  are  all  important  for  our  pur- 
pose. From  a  comparison  of  various  observations  it  thus 
became  manifest  that  the  great  displays  occurred  at 
intervals  of  about  thirty-three  years.  We  cannot  indeed 


Shooting  /Stars  155 

affirm  that  splendid  showers  of  November  meteors  have 
actually  been  observed  at  every  successive  interval  of 
thirty-three  years;  the  records  that  have  been  preserved,  or 
at  all  events  the  records  that  have  been  discovered,  are 
not  sufficiently  complete  to  enable  this  to  be  affirmed.  It 
must  be  remembered  that  if  the  night  happened  to  be 
overcast  —  and  this,  in  most  northern  climates,  is  a  circum- 
stance by  no  means  unlikely  in  the  middle  of  November  — 
then  the  shooting  stars  would  not  be  seen.  It  must  also 
be  remembered  that  as  such  displays  could  never  be  pre- 
dicted by  the  early  astronomers,  who  were  totally  ignorant 
of  their  real  character,  no  organised  arrangementcouldever 
have  been  made  for  their  observation,  and  consequently 
the  accounts  of  such  displays  which  have  been  preserved 
must  be  regarded  merely  as  fortunate  accidents.  It  may 
indeed  have  happened  that  great  showers  have  taken  place 
and  have  been  observed,  but  that  no  records  of  such  events 
have  as  yet  been  brought  to  light.  It  is  even  conceivable 
that  records  of  great  shooting  star  showers  may  still  exist 
which  have  hitherto  escaped  those  who  have  devoted 
special  attention  to  the  subject.  We  must  therefore  not 
be  surprised  if  there  are  many  gaps  in  the  history  of  these 
Leonids.  Since  the  year  902  A.D.,  when  the  earliest  of 
these  showers,  so  far  at  least  as  we  know  at  present,  was 
observed,  there  may  have  been  thirty  of  these  exceptional 
displays.  Of  these  about  a  dozen  are  known  historically ; 
as  to  the  rest,  testimony  is  silent. 

But  as  might  be  expected  we  have  had  pretty  full 
information  on  the  subject  so  far  as  the  present  century  is 
concerned.  There  was  a  great  display  of  this  particular 
shower  in  1709  and  there  was  another  great  display  in 
1833.  Thus  astronomers  were  led  to  the  expectation  that 
there  would  be  a  recurrence  of  this  phenomenon  in  I860. 
I  have  already  mentioned  how  wonderfully  this  prediction 
was  fulfilled.  It  is  the  application  of  the  same  reasoning 


156  Astronomy 

which  leads  us  to  the  expectation  that  there  will  be  a 
renewal  of  the  great  display  of  shooting  stars  at  like 
intervals  in  the  future. 

§  165.  Explanation  of  Periodic  Showers.  Since  the 
great  display  in  180G  we  have  learned  riinch  about  the 
actual  nature  of  these  little  objects  and  their  movements, 
and  we  can  now  explain  the  causes  of  their  appearance  in 
great  showers.  I  shall  here  set  forth  an  outline  of  what 
is  known  of  this  periodical  shower. 

The  Leonids  which  we  see  in  November  belong  to  a 
mighty  shoal  containing  unnumbered  myriads  of  little 
objects.  This  vast  host  sweeps  along  a  great  celestial 
highway  which  forms  an  oval  figure  nearly  two  thousand 
million  miles  long,  each,  as  a  minute  planet,  obeying  the 
attraction  of  the  Sun.  Pursuing  this  vast  track  the  shoal 
of  meteors  make  their  circuit  round  the  Sun.  Notwith- 
standing the  high  speed  with  which  these  objects  move, 
the  course  that  they  have  to  get  round  is  so  vast  that  not 
less  than  33£  years  are  required  by  them  to  accomplish 
one  complete  journey.  The  path  in  which  our  Earth 
makes  its  annual  circuit,  crosses  the  track  of  the  shooting 
stars.  In  general  the  main  shoal  of  little  objects,  although 
spread  for  many  millions  of  miles  along  the  track,  does 
not  happen  to  be  at  the  point  of  crossing  when  the  Earth 
reaches  the  same  point,  and  so  we  encounter  only  the  few 
stragglers  which  may  happen  to  lie  along  the  great  high- 
way. These  provide  for  us  the  Leonids  generally  encoun- 
tered in  the  middle  of  each  November. 

Once  every  thirty-three  years,  however,  the  principal 
part  of  the  shoal  of  these  little  bodies  arrives  at  the  point 
of  crossing  just  at  the  same  time  as  the  Earth.  The 
Earth  may  then  plough  its  way  through  the  uncounted 
myriads  and  the  result  is  a  grand  display  of  Leonids. 


CHAPTER   XIV 
STARS  AND  NEBULAE 

§  166.  The  Number  of  the  Stars.  Every  new  tele- 
scopic discovery  tends  to  give  us  larger  ideas  as  to  the 
scale  on  which  the  universe  is  built.  Our  unaided  eyes 
can  detect  at  one  view  perhaps  two  thousand  stars  of 
varied  degrees  of  magnitude  strewn  over  the  heavens. 
When  we  use  a  telescope  to  help  us,  even  though  that 
telescope  may  be  of  but  very  moderate  power,  the  number 
of  stars  is  increased  ten-  or  twenty-fold.  With  larger 
instruments  the  stars  are  increased  a  hundred-fold,  a 
thousand-fold,  even  ten  thousand-fold  or  more.  If  dis- 
carding our  visual  observation  we  employ  the  photographic 
plate,  bewildering  millions  of  stars  are  displayed  ojf  whose 
existence  we  were  previously  unaware.  The  number  of 
stars  in  such  a  picture  may  be  conjectured  from  the 
photograph  of  the  flusters  in  Perseus  referred  to  in  §  170. 

§  167.  Proper  Motion.  Many  of  the  stars  possess 
what  is  called  i>roj»jr  motion.  If  the  place  of  a  star  on  the 
heavens  be  carefully  determined  at  one  epoch  and  if  the 
place  of  the  same  star  be  again  determined  years  afterwards, 
when  allowance  has  been  made  for  the  apparent  displace- 
ment arising  from  precession,  aberration  and  nutation, 
which  affect  every  star  according  to  its  position  in  the 
157 


158  Astronomy 

heavens,  it  will  not  unf  requently  appear  that  the  position 
of  the  star  has  shifted  in  the  intervening  period.  Such 
observations  are  of  much  delicacy,  for  the  movements 
which  the  stars  make  seem  to  us  very  small  on  account 
of  their  distances,  even  though  such  movements  may  be 
intrinsically  gigantic.  A  star  might  be  moving  faster  than 
the  Earth  moves,  or  faster  indeed  than  any  planet  of  our 
system,  and  yet  in  the  course  of  a  couple  of  centuries  the 
actual  distance  which  such  a  star  would  seem  to  have 
travelled  on  the  surface  of  the  heavens  would  not  be  greater 
than  the  apparent  diameter  of  the  Moon.  If  the  star  were 
free  from  all  disturbing  effects,  such  movements  would 
proceed  uniformly,  but  if  the  star  were  acted  upon  by  the 
attraction  of  some  other  body  its  movement  would  be 
correspondingly  deranged.  In  this  way  it  will  sometimes 
happen  that  the  movements  of  the  star  will  be  appreciably 
affected  by  the  attraction  of  another  body  in  its  vicinity. 
It  is  not  the  least  necessary  that  the  attracting  body 
should  be  luminous;  all  that  is  required  is  that  it  be 
massive  enough  and  near  enough  to  the  star  to  produce 
a  considerable  effect.  In  the  movements  of  some  of  the 
stars  we  discover  unmistakable  indications  that  they  are 
influenced  by  the  attraction  of  other  bodies  in  their 
neighbourhood.  In  fact,  the  more  closely  AVC  scrutinise 
the  proper  motion  of  different  stars  the  more  do  we 
perceive  the  irregularities  with  which  such  movements 
are  affected. 

§  168.  Invisible  Stars.  The  interest  of  such  observa- 
tions is  very  great.  From  the  influence  which  such 
objects  exert  on  the  movements  of  stars  that  happen  to  be 
visible  we  learn  the  existence  of  massive  celestial  bodies 
which  we  have  never  seen  and  can  never  hope  to  see. 
Indeed  everything  we  know  about  the  stars  teaches  us  that 
the  quantity  of  matter  which  the  Universe  contains,  even 
without  going  beyond  the  telescopic  distances,  is  probably 


CLUSTER    (M.   13)    HERCULIS 
W.  E.  WILSON 


To  face  page  159 


Stars  and  Nebulae  159 

hundreds  of  times,  or  thousands  of  times,  and  it  may  be 
millions  of  times,  greater  than  the  total  mass  of  the  stars 
which  are  visible  to  us. 

§  169.  Star-Clusters.  There  is  110  spectacle  more 
astounding  to  a  student  of  the  heavens  than  that  of  a 
great  star-cluster.  It  is  well  known  that  in  this  respect 
those  astronomers  whose  lot  is  cast  in  the  Southern  hemi- 
sphere are  more  favoured  than  are  those  who  live  in 
Europe  or  Xorth  America.  The  most  glorious  star-cluster 
that  the  firmament  contains  is  that  in  the  constellation 
Centaur. 

It  sometimes  happens  that  when  an  astronomer  turns 
his  attention  towards  a  so-called  cluster  of  stars  he  will 
think  that  the  cluster  ought  to  be  described  as  a  part  of 
the  sky  where  the  stars  are  a  little  richer,  a  little  more 
abundantly  distributed,  than  usual,  rather  than  as  a 
distinct  object  evidently  indicating  a  number  of  stars 
associated  together  in  a  separate  system.  But  no  such 
feeling  is  possible  when  we  examine  such  a  wonderful 
object  as  the  great  cluster  of  the  Centaur.  Sir  John 
Herschel  studied  it  and  made  a  beautiful  drawing  of  its 
details  on  the  occasion  of  his  memorable  observation  at 
the  Cape  of  Good  Hope.  It  has  ever  since  attracted  the 
attention  of  every  astronomer  Avho  has  been  so  fortunate 
as  to  be  able  to  point  the  telescope  in  its  direction. 

§  170.  Great  Clusters  in  Hercules  and  Perseus.  In 
our  Northern  hemisphere  the  most  remarkable  of  these 
globular  clusters  is  that  in  the  constellation  of  Hercules. 
This  beautiful  celestial  object  has  been  here  represented  in 
a  reproduction  of  a  photograph  taken  by  Mr.  W.  E.  Wilson 
at  his  own  observatory  in  Westmeath.  There  is  also  a 
superb  object  of  the  kind  in  the  Swordhandle  of  Perseus. 
Fn  this  case  there  are  two  groups  of  stars  close  together, 
and  when  a  powerful  telescope  is  employed  the  multitude 
of  tlioso  stars  and  their  intrinsic  lustre  combine  to  form 


160  Astronomy 

a  most  wonderful  spectacle.  This  object  and  the  region 
of  the  Milky  Way  surrounding  it  are  here  shewn.  The 
photograph  from  which  this  picture  of  celestial  scenery 
is  reproduced  was  taken  by  Prof.  E.  E.  Barnard  at  the 
Lick  Observatory,  California. 

§  171.  The  Pleiades.  A  very  well-known  star-cluster 
is  the  famous  group  known  as  the  Pleiades.  From  the 
very  earliest  times  this  beautiful  asterism  arrested  atten- 
tion. It  was  clearly  not  the  result  of  chance  that  a  num- 
ber of  conspicuous  stars  should  be  so  closely  associated. 
It  can  easily  be  shewn  that  if  all  the  stars  in  heaven  had 
been  scattered  quite  at  random  over  the  skies,  the  proba- 
bilities would  have  been  very  many  millions  to  one  against 
the  occurrence  of  a  collection  of  stars  like  the  Pleiades 
in  the  positions  where  we  actually  find  them.  It  is  hence 
impossible  to  refuse  to  accept  the  belief  that  the  stars 
in  the  Pleiades  must  belong  to  some  organised  system. 
They  are  undoubtedly  a  group  of  associated  bodies  owning 
probably  a  common  origin  and  with  many  common  features 
in  their  structure. 

The  interest  with  which  this  famous  constellation  was 
always  regarded  became  greatly  increased  when  by  the 
extension  of  our  knowledge  we  began  to  learn  something 
of  the  sizes  and  weights  of  the  different  bodies  of  which  it  is 
composed.  So  long  as  the  stars  were  thought  to  be  merely 
little  objects  not  possessing  any  great  intrinsic  brightness, 
there  was  no  reason  to  regard  the  Pleiades  with  greater 
interest  than  belongs  to  a  collection  of  celestial  gems 
arranged  with  exqxiisite  beauty  of  detail.  But  when  it 
was  realised  that  the  group  in  question  was  in  truth  a 
collection  of  globes  sunlike  in  dimensions  and  lustre, 
the  true  importance  of  the  Pleiades  l>ecanie  recognised. 

The  individual  stars  forming  this  cluster  have  long 
engaged  the  attention  of  astronomers.  First  and  most 
brilliant  among  them  is  that  known  as  Alcyone ;  it  is  a  star 


MILKY   WAY    NEAR   CLUSTER   IN    PERSEUS 

Photographed  by  PROF.  BARNARD,  Nov.  3,  1893 

To  face  page  160 


itlne  161 

of  the  third  magnitude.  Besides  Alcyone  there  are  five 
other  important  stars  in  the  constellation.  The  brightest 
of  these  are  known  as  Electra  and  Atlas,  each  of  which  is 
of  about  the  fourth  magnitude.  In  the  descending  scale  of 
lustre  we  have  Maia,  near  the  fifth ;  Merope  and  Taygeta 
are  much  about  the  same.  The  seventh  star  of  the 
Pleiades  is  Celaeno ;  it  is  only  occasionally  to  be  seen  with 
the  unaided  eye.  Acute  vision  will,  however,  shew  many 
other  stars  in  this  famous  group.  Some  observers  gifted 
with  exceptional  vision  have  been  able  to  see  twelve  of  the 
Pleiades.  Xo  doubt  there  are  quite  this  number  of  stars 
in  the  group  bright  enough  to  be  visible  to  most  ordinary 
eyes,  if  they  were  presented  as  isolated  spots  on  the  dark 
sky,  but  the  fact  that  the  Pleiades  is  so  crowded  increases 
the  difficulty  of  discriminating  the  fainter  points. 

With  the  slightest  telescopic  power  the  number  of 
visible  Pleiades  is  seen  to  be  enormously  increased.  Ko 
less  than  sixty-nine  stars  can  be  seen  with  a  very  small 
instrument  which  can  easily  be  held  in  the  hand.  But  it 
must  not  be  supposed  that  the  number  of  stars  just 
referred  to  includes  all  those  in  the  group.  Their  number 
is  indeed  vastly  greater.  With  every  increase  in  the  power 
of  the  telescope  more  and  more  stars  are  brought  within 
the  range  of  vision.  And  what  the  most  powerful  telescope 
is  able  to  render  visible  to  the  eye  is  vastly  transcended 
by  the  results  obtained  by  taking  a  photograph  of  long 
exposure.  The  brothers  Henri,  in  Paris,  have  obtained 
more  than  two  thousand  star-images  on  a  single  plate 
directed  to  the  Pleiades.  iNor  is  there  the  least  reason  to 
think  that  the  full  tale  of  stars  in  the  group  has  been 
even  yet  ascertained.  Each  increase  in  the  sensibility  of 
the  plate  or  in  the  duration  of  its  exposure  invariably 
brings  with  it  an  increased  number  of  the  stars  which  are 
represented. 

We  shall  explain  in  §  185  how  the  distances  of  the 


162  AstTonomy 

stars  are  determined.  It  will  however  frequently  happen 
that  the  stars  are  so  remote  that  the  method  becomes 
inapplicable.  All  we  can  then  do  is  to  determine  a  certain 
minimum  distance  which  we  can  say  is  certainly  exceeded. 
And  in  this  case  all  we  know  about  the  distance  of  the 
Pleiades  amounts  to  the  statement  that  this  group  must  be 
millions  of  times  as  remote  from  our  Earth  as  is  the  Sun. 
This  leads  to  a  very  interesting  conclusion.  We  can 
estimate  what  the  brightness  of  our  Sun  would  be  if  it 
were  transferred  to  a  distance  such  as  that  which  we  know 
must  be  at  least  the  distance  of  the  Pleiades.  It  seems 
perfectly  certain  that  under  such  circumstances  the  Sun 
would  send  us  less  light  than  the  very  faintest  Pleiad 
which  the  keenest  eye  can  detect.  We  do  not  indeed 
assert  that  the  Sun  is  larger  than  every  one  of  the  two 
thousand  and  upward  stars  of  which  the  Pleiades  is  com- 
posed. It  is  however  certain  that  Alcyone  and  probably 
some  of  the  other  brilliant  gems  of  the  constellation  must 
be  hundreds  of  times  more  lustrous  than  our  own  orb  of 
day.  A  planet  revolving  around  Alcyone  in  an  orbit  as 
great  as  that  which  Neptune,  the  outermost  of  our  planets, 
describes  around  the  Sun,  would  only  circulate  through 
a  wholly  insignificant  portion  of  the  mighty  cluster;  so 
close  indeed  would  it  appear  to  Alcyone  that,  even  if  it 
were  a  bright  object  like  Alcyone  itself,  a  good  telescope 
would  be  required  to  see  the  two  objects  apart. 

§  172.  Spectra  of  the  Stars  in  the  Pleiades.  Modern 
spectroscopic  research  has  brought  evidence  of  a  very 
striking  character  to  shew  the  common  origin  and  the 
common  physical  character  of  the  stars  in  the  Pleiades. 
It  is  known  of  course  that  the  light  in  the  spectrum  of  a 
star  is  eminently  characteristic  of  that  star.  Indeed  it  is 
doubtful  if  any  two  stars  in  the  heavens  would  manifest 
exactly  the  same  spectrum  if  we  were  able  to  see  all  the 
lines  that  the  spectrum  of  each  contained.  Professor 


DUMB-BELL   NEBULA 

W.  E.  WILSON 


To  face  Page  163 


Stars  and  Nebulae  163 

Pickering,  who  has  devoted  himself  with  such  skill  to  the 
study  of  the  spectra  of  the  stars,  has  examined  the  light 
which  is  emitted  to  us  from  the  principal  stars  of  the 
Pleiades.  It  has  been  found  that  their  spectra  are  sub- 
stantially of  one  type.  This  is  a  confirmation  of  the 
statement  that  the  stars  in  the  Pleiades  form  an  associated 
group. 

§  173.  Nebulae.  There  are  a  number  of  objects  in 
the  heavens  which  are  termed  nebulae.  One  or  two  of 
these  are  visible  to  the  unaided  eye,  but  the  rest,  to  the 
number  of  some  seven  thousand  or  more,  require  telescopic 
power  and  often  telescopes  of  very  high  power  indeed. 
Many  of  these  objects  are  strictly  gaseous  in  their  character. 
By  the  method  of  spectrum  analysis  it  has  been  shewn  that 
they  contain  hydrogen  as  well  as,  in  all  probability,  certain 
other  substances. 

One  of  the  most  remarkable  of  these  objects  is  well 
known  to  every  astronomical  observer.  This  is  the  Great 
Xebula  in  the  Swordhandle  of  Orion  which  seems  to  be 
a  huge  chaotic  mass  of  glowing  gas.  Although  this  object 
lias  been  under  observation  for  200  years  and  whole  books 
have  been  written  on  the  phenomena  it  presents,  yet  we 
are  still  a  very  long  way  indeed  from  a  complete  know- 
ledge of  the  conditions  under  which  it  exists. 

We  here  reproduce  a  remarkable  photograph  taken  by 
Mr.  W.  E.  Wilson.  Tli i  s  i s  the  "  Dumb-bell "  in  Vulpecula, 
In  it  we  see  an  approach  to  a  symmetrical  form,  the  mass 
being  bounded  at  its  northern  and  southern  limits  by 
approximately  circular  arcs  of  increased  brightness. 

In  the  constellation  of  Andromeda  a  faint  patch  of 
indistinct  brightness  can  just  be  detected  with  the  naked 
eye  on  a  clear  night.  This  is  the  Great  Nebula  in  Andro- 
meda and  is  the  only  true  nebula  which  was  known  before 
the  days  of  the  telescope.  This  has  always  been  one  of 
the  most  interesting  telescopic  objects  of  its  class  in  the 


164  Astronomy 

heavens,  but  certain  features  were  never  properly  under- 
stood until  the  photographic  plate  was  applied  by  Dr.  Isaac 
Koberts  to  supplement  the  powers  of  the  human  eye. 

Then  it  was  seen  that  it  consisted  of  an  enormous  mass 
of  glowing  material  surrounded  by  separate  rings  of  a 
similar,  but  less  intensely,  bright  appearance.  We  seem 
to  be  looking  down  obliquely  on  the  plane  in  which  the 
rings  are  arranged  so  that  they  appear  to  be  projected  into 
somewhat  elongated  ellipses.  A  photograph  of  this  nebula 
shewing  also  Holmes'  Comet  was  taken  by  Prof.  Barnard 
on  Nov.  10,  1892,  and  is  here  reproduced. 

Astronomers  have  not  yet  succeeded  in  determining 
how  far  from  the  Earth  this  wonderful  object  must  be,  and 
consequently  it  is  impossible  to  say  with  any  pretence  to 
precision  what  its  dimensions  can  be.  But  we  may  set  a 
lower  limit  beyond  which  it  must  lie,  since  otherwise  it 
would  have  afforded  evidence  of  an  annual  displacement 
which  would  have  enabled  us  to  measure  its  distance.  We 
may  thus  with  confidence  say  that  the  great  nebula  in 
Andromeda  is  at  least  100,000  times  the  Sun's  distance 
from  us.  From  the  apparent  size  of  the  object  as  seen  in 
the  sky  we  know  that  its  exterior  diameter  cannot  be  less 
than  ^-jth  part  of  its  distance.  Hence  it  follows  that  this 
nebula  must  measure  at,  least  400,000  millions  of  miles  from 
side  to  side,  or  more  than  70  times  the  diameter  of  the  orbit 
of  Neptune.  This  is  the  very  lowest  estimate,  and  we  should 
probably  not  be  erring  on  the  side  of  excess  if  we  stated 
that  the  true  dimensions  are  quite  ten  times  as  much  as 
the  figures  given. 

One  of  the  most  interesting  discoveries  recently  made, 
in  connexion  with  nebulae,  has  revealed  to  us  that  some  of 
these  objects,  including  indeed  the  very  greatest  nebulae, 
are  totally  invisible.  They  are  not  to  be  seen  by  the 
eye  even  with  the  help  of  the  best  telescopes,  and  can 
only  be  discerned  by  the  photographic  plate.  A  remarkable 


THE   GREAT   NEBULA    IN   ANDROMEDA   AND 
HOLMES'   COMET 

Photographed  by  PROF.  BARNARD,  Nov.  10,  1892 

To  face  page  164 


Stars  and  Nebulae  165 

instance  of  a  nebula  of  this  kind  is  presented  in  connexion 
with  the  group  of  stars  in  the  Pleiades.  When  a  photo- 
graph is  taken  with  an  exposure  of  not  less  than  an  hour,  a 
nebula  with  considerable  luminous  density  and  of  enormous 
volume  is  seen  to  cover  the  whole  group  of  stars.  The 
nebula  is,  however,  of  very  variable  brightness  in  different 
parts  of  the  cluster.  When  such  a  plate  as  this  was  first 
taken  it  was  hard  to  read  aright  the  tale  that  it  unfolded. 
It  might  at  first  be  natural  to  attribute  the  phenomena 
revealed  on  the  plate  to  some  blemishes  in  the  process  of 
development,  or  to  some  accidental  disorder  in  the  plate 
itself.  But  when  one  photograph  after  another  displayed 
precisely  the  same  luminous  configuration,  when  telescopes 
of  very  varying  sizes  invariably  reproduced  the  nebula 
with  the  same  general  characteristics,  when  the  nebula 
was  alike  apparent  on  photographs  obtained  by  telescopes 
which  refracted,  and  by  telescopes  which  reflected,  then 
it  became  manifest  that  the  observed  phenomena  could 
not  be  explained  away  as  arising  from  any  accidents  in 
the  operation  or  from  any  defects  in  the  plate. 

The  lesson  that  we  thus  learn  with  regard  to  the 
Pleiades  is  very  instructive.  If  the  evidence  had  seemed 
insufficient  to  convince  us  hitherto  that  these  stars  really 
constituted  a  group  bound  together  by  physical  bonds, 
the  facts  now  brought  forward  would  have  dispelled  such 
a  notion.  The  nebula  includes  within  its  mighty  compass 
the  stars  of  the  Pleiades.  It  would  be  difficult  to  sup- 
pose that  this  nebula  Avas  an  isolated  volume  of  vapour 
which  happened  by  some  chance  to  be  located  directly  on 
the  line  connecting  the  Solar  System  and  the  Pleiades.  It 
would  not  be  reasonable  to  suppose  that  the  great  fire- 
cloud  had  been  casually  projected  on  a  remarkable  group 
of  stars  in  the  background.  It  would  be  just  as  unreason- 
able to  suppose  that  the  group  of  stars  has  been  casually 
projected  on  the  great  nebula  stretched  as  a  curtain  behind 


166 

it.  We  cannot  withhold  our  assent  from  the  belief  that 
the  nebula  and  the  group  of  stars  together  form  one 
majestic  object  in  the  firmament. 

§  174.  Double  Stars.  It  frequently  happens  that  a 
star  which  to  the  unaided  eye  would  appear  as  a  single 
object  is  shewn  in  the  telescope  to  consist,  of  two  stars 
close  together.  These  objects  are  known  as  double  stars, 
and  their  number  is  such  that  it  is  almost  impossible  to 
suppose  that  the  contiguity  in  which  they  lie  is  always,  or 
even  frequently,  accidental.  Every  one  who  is  accustomed 
to  use  a  telescope  is  familiar  with  many  of  those  double 
stars,  which  form  indeed  some  of  the  most  interesting 
telescopic  spectacles  that  the  heavens  have  to  shew.  Some 
of  these  are  easy  objects,  that  is  to  say,  a  very  moderate 
degree  of  telescopic  power  will  suffice  to  demonstrate  the 
astonishing  fact  that  the  star,  which  to  the  unaided  eye 
looks  like  an  ordinary  star  consisting  of  a  single  globe, 
is  in  reality  a  pair  of  associated  globes,  so  close  together 
that  the  eye  is  not  able  to  distinguish  them  separately 
until  the  magnifying  power  of  the  telescope  has  been  called 
into  requisition.  In  many  cases  these  pairs  of  stars  are  so 
close  together  that  a  demand  has  to  be  made  on  the  very 
.highest  powers  which  can  be  applied  to  the  telescope  in 
order  to  exhibit  the  two  objects  as  separate  points.  Indeed 
it  is  well  known  that  the  criterion  of  the  performance  of  a 
telescope  is  generally  given  by  the  capability  which  it  shews 
for  dividing  the  two  components  of  a  double  star.  We 
may  give  one  illustration  to  shew  the  apparent  proximity 
in  which  the  two  components  of  a  double  star  will  some- 
times lie  in  the  heavens.  A  good  telescope,  such  as  can 
now  be  found  in  all  public  observatories  and  in  many 
private  hands,  will  suffice  to  separate  two  nearly  equal 
stars  if  the  angular  distance  between  them  be  one  second. 
With  the  help  of  telescopes  of  the  very  highest  class  pairs 
of  stars  which  lie  much  closer  still  can  be  separated. 


Stars  and  Nebulae  167 

Of  course  it  will  be  understood  that  when  we  speak 
of  the  proximity  of  the  two  components  of  a  double  star 
we  mean  that  they  appear  very  close  together  on  the  sky 
and  the  distance  by  which  the  pair  is  separated  from  the 
terrestrial  observer  must  be  borne  in  mind.  Even  the 
closest  pair  which  has  ever  been  separated  by  the  most 
powerful  telescope  must  still  have  a  distance  of  many 
million  miles  between  its  components.  Indeed  we  know 
that  if  an  observer  on  one  of  these  stars  could  perceive 
the  Earth,  the  distance  between  it  and  the  Sun  would 
appear  to  him,  from  the  position  in  which  he  is  placed, 
quite  as  small  as  do  the  apparent  distances  between 
the  two  components  of  many  of  the  double  stars  appear 
to  us. 

We  should,  however,  mention  that  many  of  the  pairs 
of  objects,  which  are  usually  spoken  of  as  double  stars,  are 
not  to  be  regarded  as  forming  systems  of  the  same  character 
as  those  which  we  have  been  here  describing  and  which  are 
known  as  binary  systems.  It  may  of  course  happen  that 
two  stars  which  are  in  no  way  connected  lie  so  nearly  in 
line  with  the  Earth,  that  when  viewed  from  the  Earth  they 
appear  close  together  in  the  sky,  though  as  a  matter  of  fact 
one  of  these  stars  may  be  ten  times,  or  a  hundred  times,  as 
far  from  us  as  the  other.  Objects  of  this  class  are  not 
spoken  of  as  a  binary  pair.  Their  proximity  is  merely 
casual,  depending  on  the  accident  that  the  line  joining 
them  is  directed  towards  the  Earth.  Such  pairs  of  stars 
are  of  course  not  physically  related,  but  it  is  often  not  easy 
to  distinguish  such  pairs  from  genuine  binaries.  There  is, 
however,  one  method  sometimes  available  for  discriminat- 
ing between  a  pair  of  stars  which  are  merely  optically 
double,  and  a  genuine  binary  pair.  As  we  have  mentioned 
already,  many  of  the  stars  of  all  classes  are  animated  by 
what  are  called  proper  movements.  As  each  such  star  is 
borne  along  it  drifts  relatively  to  the  other  stars  which  hap- 


168  Astronomy 

pen  to  lie  contiguous  to  its  track.  In  the  course  of  many 
years  a  star  endowed  with  considerable  proper  motion  may 
thus  drift  away  from  another  star  which  seemed  originally 
to  lie  close  to  it.  If  however  the  two  members  of  a  pair 
of  stars  were  physically  associated,  then  in  the  drift  of  one 
it  would  necessarily  be  accompanied  by  the  other  and  the 
two  would  thus  drift  in  company  relatively  to  the  other 
stars  in  the  vicinity.  In  this  way  we  are  often  provided 
with  a  criterion  by  which  a  binary  star  may  be  distin- 
guished from  a  pair  which  is  merely  optically  double. 

§  175.  Binary  Stars.  In  the  case  of  a  genuine  binary 
pair,  of  which  the  stars  not  only  seem  to  be,  but  are  in 
fact,  close  together,  —  close  that  is  to  say  as  compared  to 
the  distance  by  which  we  are  separated  from  them, — they 
revolve  about  each  other  in  consequence  of  their  mutual 
attraction.  The  movements  are  indeed  conducted  in  con- 
formity with  Kepler's  laws,  and  in  the  case  of  some  of  the 
more  rapidly  moving  binaries  it  is  extremely  interesting 
to  watch  the  way  in  which  the  relative  positions  of  the 
two  stars  are  observed  to  shift  year  after  year.  By  making 
a  series  of  measurements  of  their  relative  positions,  we  are 
frequently  enabled  to  determine  the  track  of  one  of  these 
objects  around  the  other,  and  to  ascertain  the  number  of 
years  in  which  its  revolution  is  accomplished.  In  some 
cases  the  period  in  which  the  stars  revolve  does  not  exceed 
six  or  seven  years.  In  other  cases,  however,  the  periodic 
time  mounts  up  to  centuries. 

§  176.  Mass  of  a  Binary.  One  of  the  most  interest- 
ing consequences  that  can  be  deduced  from  observations 
of  the  binary  stars  is  the  determination  of  the  masses 
of  the  stars  forming  such  a  pair.  If  we  know  the  dimen- 
sions of  their  orbits  as  well  as  the  periodic  times  in  which 
the  revolutions  are  accomplished,  we  are  able  to  tell  the 
masses  concerned  in  the  attraction,  just  as  we  are  able 
to  find  the  mass  of  the  planet  from  the  revolution  of  its 


Stars  and  Nebulae  169 

satellite.  In  order  to  obtain  the  actual  size  of  the  orbit  we 
must  know  the  distance  of  the  pair  from  the  Sun,  and  as 
this  is  known  for  only  a  very  limited  number  of  stars,  this 
method  is  not  at  present  of  very  wide  application.  Seeing 
that  the  stars  are  suns  we  naturally  desire  to  utilise  the 
information  thus  gained  to  make  a  comparison  between 
the  masses  of  the  stars  and  the  mass  of  our  own  Sun. 
The  circumstances  of  a  few  binary  stars  permit  us  to  deter- 
mine their  weights.  Some  of  the  binary  stars  are  found 
to  be  about  equal  in  weight  to  the  Sun  while  others  are 
by  no  means  so  heavy.  Many  are,  however,  far  heavier 
than  the  great  orb.  We  may  in  fact  regard  our  Sun  as 
a  star  of  average  magnitude,  in  so  far  at  least  as  the 
information  given  to  us  by  double  stars  is  concerned. 

§  177.  Colours  of  Double  Stars.  A  beautiful  feature 
connected  with  the  double  stars  is  found  in  the  circum- 
stance that  the  component  orbs  are  frequently  tinged 
with  different  hues.  In  many  cases  the  tints  of  the  two 
stars  are  beautifully  contrasted,  and  sometimes  the  colours 
may  be  said  to  be  complementary.  Among  the  most 
famous  of  coloured  pairs  is  the  object  known  as  Beta  in 
the  constellation  of  the  Swan.  One  of  its  components 
possesses  a  beautiful  topaz  colour,  while  the  other  is  of  an 
emerald  hue.  Indeed  in  several  cases  the  lesser  of  the  two 
components  of  a  double  star  displays  a  bluish  or  a  violet 
tinge,  and  this  is  the  more  remarkable  when  it  is  observed 
that,  unless  in  association  with  another  star  forming  a 
binary  pair,  blue  stars  are  almost  unknown. 

§  178.  Triple  Stars.  Much  additional  interest  is  often 
imparted  by  the  circumstance  that  one  of  the  two  com- 
ponents will  itself  be  found  to  be  a  double  star.  There 
is  a  famous  pair  of  this  class  in  Andromeda  (Gamma 
Andromedae),  the  smaller  component  of  which  possesses 
an  exquisite  blue  colour.  I>y  a  very  good  telescope  this 
component  can  be  itself  resolved  into  two  little  blue  stars 


170  Astronomy 

so  close  as  to  be  incapable  of  being  distinguished  sepa- 
rately by  any  instrument  which  is  not  first-rate. 

§  179.  Multiple  Stars.  In  many  cases  there  are  sev- 
eral stars  associated  in  one  system,  and  the  complexity 
of  the  movements  in  groups  of  this  kind  would  baffle  the 
skill  of  the  most  consummate  mathematician.  Up  to  the 
present,  however,  mathematicians  have  had  but  little 
opportunity  for  even  attempting  to  investigate  the  me- 
chanical subtleties  of  these  elaborate  stellar  systems.  Not 
until  many  more  observations  have  been  accumulated  will 
it  be  possible  to  attempt  any  investigations  of  this  kind 
with  hopes  of  success. 

§  180.  Spectroscopic  Binaries.  One  of  the  most  strik- 
ing discoveries  Avhich  has  been  made  in  connexion  with 
double  stars  in  recent  years  teaches  us  that  there  are 
multitudes  of  binary  pairs  whose  components  are  so  close 
together  that  there  is  not  the  slightest  chance  of  our  ever 
being  able  to  see  them  separately,  even  if  our  telescopes 
were  hundreds  of  times  more  powerful  than  those  which 
are  at  present  available.  That  certain  stars,  which  all  our 
telescopes  would  shew  as  single,  must  be  double  in  reality 
has  however  been  demonstrated  from  the  fact  that  when 
the  light  received  from  them  has  been  passed  through  the 
spectroscope  to  a  photographic  plate  it  gives  unmistakable 
evidence  of  having  emanated  not  from  a  single  source  but 
from  two  contiguous  bodies  in  rapid  motion  relatively  to 
each  other.  This  is  shewn  by  the  fact  that  certain  lines 
in  the  spectrum  present  themselves  as  double  when  one 
of  the  stars  is  approaching  and  the  other  is  receding,  while 
when  both  stars  are  moving  across  the  line  of  vision  the 
two  lines  coincide.  In  this  manner  it  has  been  demon- 
strated that  there  are  pairs  of  suns  so  close  together  that 
they  revolve  around  each  other  in  a  few  days,  and  from.an 
examination  of  a  sufficient  series  of  such  photographs  it 
has  been  possible  to  learn  the  dimensions  of  the  orbit  in 


Stars  and  Nebulae  171 

which  these  components  move  as  well  as  their  periodic 
times  and  thus  to  ascertain  their  weight.  This  branch  of 
research  is  at  present  merely  in  its  infancy,  but  it  seems  to 
indicate  that  numbers  of  objects  which  still  appear  to  be 
merely  single  stars  must  ultimately  disclose  themselves 
as  double. 


CHAPTER   XV 

CAUSES   AFFECTING   THE   APPARENT    PLACES   OF 

THE  STARS 

§  181.  Aberration  of  Light.  Besides  the  diurnal  move- 
ment of  the  stars,  there  are  other  apparent  movements 
which  are  of  the  utmost  importance  alike  in  the  Theory 
of  Astronomy  and  in  the  work  of  the  Observatory. 

One  of  the  most  important  is  that  Avhich  was  discovered 
in  1726  by  the  illustrious  astronomer  Bradley  from  his 
observations  of  the  star  y  Draconis.  It  followed  from  his 
study  of  that  star,  that  it  seemed  to  describe  a  small 
elliptic  path  on  the  surface  of  the  heavens.  The  length 
of  the  axis  of  the  ellipse  was  about  forty  seconds  of  arc, 
and  the  period  required  to  complete  a  circuit  was  one  year 
exactly.  Further  study  shewed  that  other  stars  had  similar 
movements.  A  star  at,  or  near,  the  pole  of  the  ecliptic 
moved  in  a  circular  track.  As  the  position  of  the  star  was 
nearer  to  the  ecliptic,  the  ellipse  became  more  and  more 
eccentric  until  in  the  case  of  a  star  actually  in  the  ecliptic 
the  movement  of  the  star  was  no  more  than  an  oscillation 
to  and  fro  in  a  straight  line.  In  every  case  however  the 
length  of  the  major  axis  was  the  same,  i.e.  40".  In  every 
case  the  major  axis  was  parallel  to  the  ecliptic,  and  in 
every  case  the  period  is  exactly  one  year. 

These  circumstances  pointed  out  that  an  explanation 
of  the  apparent  movements  must  be  in  some  way  con- 
nected with  the  annual  motion  of  the  Earth  in  its  orbit. 
172 


Apparent  Places  of  the  Stars  173 

By  a  happy  stroke  of  genius  Bradley  saw  what  the 
explanation  must  be,  and  his  discovery  is  referred  to  as 
that  of  the  aberration  of  Light. 

Light  travels  at  a  speed  which  is  about  186,000  miles  a 
second,  i.e.  about  10,000  times  as  fast  as  the  progress  of 
the  Earth  in  its  orbit.  If  the  velocity  of  light  had  been 
infinitely  greater  than  the  velocity  of  the  Earth,  then  the 
telescope  pointed  to  the  apparent  place  of  the  star  would 
be  pointed  to  the  true  place.  As  however  the  velocity  of 
light  is  not  infinitely  greater  but  only  10,000  times  greater 
than  the  velocity  of  the  Earth  in  its  track,  the  telescope 
directed  to  the  star  has  to  be  directed  to  a  neighbouring 
point  between  the  true  place  of  the  star  and  the  point  on 
the  celestial  sphere  towards  which  the  Earth  is  at  the 
moment  urging  its  way.  This  apparent  shift  in  the  place 
of  the  star  can  be  shewn  to  produce  all  the  phenomena 
observed  under  the  title  of  aberration. 

§  182.  Precession  of  the  Equinoxes.  In  connexion 
with  the  places  of  the  stars  we  have  to  consider  the 
phenomenon  discussed  in  §  104  which  is  known  as  the 
precession  of  the  equinoxes.  We  have  seen  how  the  right 
ascension  of  a  star  is  to  be  measured  by  the  angular  dis- 
tance between  the  great  circle  passing  through  the  pole 
and  that  star,  and  the  great  circle  passing  through  the 
pole  and  the  equinox.  Let  us  suppose  that  observations 
of  this  kind  are  repeated  from  year  to  year,  and  to  take  a 
particular  case,  I  shall  suppose  that  of  the  star  Sirius. 
If  the  right  ascension  of  Sirius  be  determined  year  after 
year  it  will  be  found  that  the  right  ascension  increases  at 
the  rate  of  2-65  seconds  per  annum.  This  increase  is  not 
quite  uniform.  But  that  is  its  average  amount,  and  the 
increase  is  continually  going  on  in  the  same  direction. 
If  we  take  a  considerable  period  of  time  as  an  illustra- 
tion, then  we  learn  that  in  a  hundred  years  the  right 
of  Sirius  is  increased  by  265  seconds,  that  is 


174  Astronomy 

by  over  four  minutes.  Kow  this  movement  must  be 
due  to  an  alteration  either  in  Sirius  or  in  the  equinox, 
for  we  have  said  that  the  right  ascension  is  the  angle 
which  Sirius  and  the  equinox  subtend  at  the  pole  of  the 
celestial  sphere.  It  is  however  found  by  comparing  Sirius 
with  other  stars  that  Sirius  remains  practically  stationary 
with  regard  to  them,  and  hence  we  are  assured  that  the 
vernal  equinox  must  be  itself  in  motion  amongst  the  stars. 
The  tendency  is  always  for  the  right  ascension  to  increase, 
which  shews  that  the  vernal  equinox  is  always  receding 
a  little,  so  as  to  make  it  ever  tend  to  come  earlier  and 
earlier  on  the  meridian.  This  is  the  phenomenon  which 
we  call  the  precession  of  the  equinox.  I  have  men- 
tioned the  star  Sirius,  but  a  like  result  would  have 
been  obtained  by  considering  the  movements  of  any 
other  star.  In  short  all  the  stars  would  agree  in  shew- 
ing that  there  was  this  continuous  movement  constantly 
bringing  the  equinox  around  amongst  the  stars  from  east 
to  west. 

The  equinox  is  however  defined  to  be  the  point  in 
which  the  ecliptic  and  the  equator  intersect  each  other. 
If  therefore  the  equinox  is  in  motion  this  point  of  inter- 
section of  the  ecliptic  and  the  equator  is  in  motion.  It 
therefore  follows  necessarily  that  either  the  great  circle 
which  we  call  the  equator,  or  the  great  circle  which  we  call 
the  ecliptic,  or  it  may  happen  to  be  both  those  circles,  is  in 
movement.  Let  us  consider  which  of  these  three  supposi- 
tions is  the  case.  So  far  as  the  ecliptic  is  concerned  we  are 
assured  that  it  has  not  any  motion  which  will  avail  us  for 
explaining  the  phenomenon  in  question,  for  the  distances 
of  all  the  stars  as  measiired  from  it  are  found  to  remain 
practically  unchanged.  The  track  of  the  Sun  as  laid 
down  through  the  stars  does  not  alter  appreciably.  We 
are  therefore  reduced  to  attributing  the  movements  known 
as  precession  to  a  shift  of  the  equator  along  the  ecliptic. 


Apparent  Places  of  the  Stars  175 

§  183.  Change  in  the  Obliquity.  With  regard  to  the 
movements  of  the  equator  it  may  alter  either  by  simply 
rotating  around  on  the  ecliptic,  constantly  preserving 
the  same  angle  of  inclination  thereto,  or  it  would  be 
possible  for  the  equator  to  alter  its  obliquity  to  the 
ecliptic,  or  it  would  be  possible  for  the  equator  to  have 
movements  of  both  kinds.  But  it  is  certainly  demonstrated 
that  the  obliquity  of  the  ecliptic  does  not  much  alter. 
The  obliquity  is  determined  by  measuring  the  polar  distance 
of  the  Sun  at  midsummer  and  subtracting  that  from  ninety 
degrees,  and  it  is  found  that  the  obliquity,  though  not 
absolutely  constant,  still  varies  very  slightly.  Its  changes 
in  a  period  of  twenty  years  would  not  amount  to  a  seven- 
thousandth  part  of  its  total  value.  We  may  therefore 
from  our  present  point  of  view  treat  the  obliquity  of  the 
ecliptic  as  a  constant  quantity.  We  are  therefore  led  to 
the  conclusion  that  the  precession  of  the  equinoxes  must 
be  due  to  the  movements  of  the  equator  around  the  eclip- 
tic. A  further  examination  shews  that  the  amount  of 
the  precession  is  50-24  seconds  of  arc  annually.  This  is 
the  annual  change  in  the  position  of  the  intersection  of  the 
equator  with  the  ecliptic  measured  along  the  ecliptic. 
This  movement  is  no  doubt  a  slow  one,  for  it  is  easy  to 
shew  that  a  period  of  very  little  less  than  twenty-six 
thousand  years  must  elapse  to  enable  the  equinox  while 
moving  at  this  rate  to  make  a  complete  circuit  of  the 
ecliptic. 

§  184.  Precessional  Movement  of  the  Pole.  We  can 
appreciate  more  clearly  the  nature  of  this  movement  by 
thinking  of  the  pole  of  the  equator,  that  is,  of  the  celes- 
tial pole  of  the  heavens,  rather  than  of  the  equator  itself. 
Since  the  angle  between  two  great  circles  is  equal  to  the 
angle  between  their  poles,  it  follows  that  the  distance  of 
the  pole  of  the  ecliptic  from  the  pole  of  the  equator  is 
equal  to  the  obliquity  of  the  ecliptic.  As  the  ecliptic  is 


176 


Astronomy 


a  fixed  circle,  so  the  pole  of  the  ecliptic  is  a  fixed  point 
amongst  the  stars.  And  as  the  obliquity  is  constant  it 
follows  that  the  pole  of  the  equator  must  be  describing 
a  small  circle  on  the  celestial  sphere  around  the  pole  of 
the  ecliptic  as  its  centre.  The  radius  of  this  circle,  that 
is  the  obliquity  of  the  ecliptic,  is  23°  27'  8",  and  the 
pole  completes  its  revolution  around  the  circle  in  a  period 
of  about  twenty-six  thousand  years. 

Notwithstanding  the  slowness  of  this  motion,  it  is  yet 
capable  of  producing  most  striking  effects  on  the  arrange- 
ments of  the  constellations  relatively  to  the  pole,  as  may 
be  seen  from  Fig.  23,  which  represents  the  path  of  the  pole 


Fig.  23.     Precessional  Movement  of  the  Pole. 


Apparent  Flaw*  of  the  Stars  177 

among  the  constellations.  For  instance,  at  present  the 
star  of  the  second  magnitude  which  we  call  the  Pole  Star 
is  within  a  degree  and  a  quarter  of  the  pole.  But  the 
nearness  of  the  star  and  the  pole  is  merely  fortuitous. 
The  pole  is  constantly  moving  on,  and  at  present  it  so 
happens  that  the  pole  of  the  heavens  is  approaching  the 
Pole  Star.  In  a  couple  of  centuries  the  Pole  Star  will  in 
fact  serve  its  purpose  even  still  better  than  it  does  at 
present,  because  it  will  be  within  half  a  degree  of  the 
true  pole.  After  this,  however,  the  true  pole  will  begin  to 
separate  from  the  Pole  Star,  and  it  may  be  mentioned 
that  in  about  twelve  thousand  years  the  pole  will  have 
advanced  to  the  opposite  side  of  the  circle  which  forms  its 
orbit,  and  it  will  then  have  advanced  to  within  about  five 
degrees  of  the  bright  star  Vega,  which  will  accordingly 
serve  as  a  Pole  Star  for  many  of  the  purposes  for  which 
our  Pole  Star  is  used  at  present. 

§  185.  Annual  Parallax.  Connected  with  the  annual 
movement  of  the  Earth  around  the  Sun  is  the  phe- 
nomenon of  annual  parallax  Avhich  is  displayed  by  some 
of  the  stars.  Seeing  that  the  Sun's  distance  is  about 
ninety-three  millions  of  miles,  it  follows  that  an  observer 
on  the  Earth  who  views  a  star  on  a  particular  day  and  again 
on  this  day  six  months  does  so  from  the  opposite  ends  of  a 
line  which  is  a  hundred  and  eighty-six  millions  of  miles  in 
length.  So  great  a  displacement  in  the  position  of  the 
observer  might  be  expected  to  carry  with  it  a  corresponding 
displacement  in  the  apparent  direction  of  the  stars.  If  no 
great  displacement  can  be  perceived  in  the  place  of  the 
star  then  all  that  can  lie  said  is  that  the  distance  of  the 
star  is  so  vast  that  the  diameter  of  the  Earth's  orbit  is 
merely  an  inconsiderable  magnitude  in  reference  thereto. 
This  question  has  been  studied  very  carefully  and  labori- 
ously, because  it  pi-ovides  us  with  the  only  possible  means 
of  solving  the  problem  of  the  determination  of  the  distances 


178  Astrono'intj 

of  the  stars.  We  are  to  imagine  a  triangle  whereof  the 
vertex  is  a  star  and  the  base  a  diameter  of  the  Earth's 
orbit.  The  observer  when  he  is  at  one  end  of  that  diameter 
measures  the  angle  between  the  Sun  and  the  star.  When 
he  is  at  the  other  side  of  the  orbit  he  measures  again  the 
angle  between  the  Sun  and  the  star.  The  length  of  the 
base  being  known  and  the  two  angles  of  the  base  being 
determined,  then  the  solution  of  a  triangle  ought  to  give 
the  position  of  the  star.  Such  is  in  outline  the  method  for 
determining  the  distance  of  a  star.  If,  however,  it  so 
happened  that  the  distance  of  the  star  was  enormously 
great,  then  this  triangle  would  be  a  very  ill-conditioned 
one.  The  two  sides  of  the  triangle  would  be  nearly 
parallel  and  the  observed  angles  would  be  very  nearly  sup- 
plemental. Under  such  circumstances  a  minute  error  in 
observing  the  angles  would  entail  an  enormous  difference 
in  the  concluded  distance  of  the  star.  This  is  just  the 
difficulty  that  arises.  The  stars  one  and  all  are  so  far  off 
that  even  under  the  most  favourable  circumstances  the 
two  long  sides  of  this  triangle  are  each  at  least  two 
hundred  thousand  times  as  long  as  the  base.  Nor  does 
the  difficulty  end  here.  The  direct  measurement  of  the 
angles  from  the  Sun  to  the  star  is  attended  with  such 
practical  difficulties  that  it  would  involve  errors  very 
much  larger  than  the  third  angle  of  the  triangle,  and 
consequently  this  method  would  break  down. 

But  by  a  modification  of  the  process  it  has  been  found 
possible  in  several  cases  to  determine  the  distances  of  stars. 
Let  us  suppose  that  we  have  two  stars  apparently  close 
together,  that  is  to  say,  lying  within  the  same  field  of  the 
telescope,  but  that  in  reality  one  of  these  stars  is  very 
much  more  distant  than  the  other,  suppose  ten  times  as 
far.  The  change  of  the  place  of  the  observer  from  one  end 
of  the  diameter  of  the  Earth's  orbit  to  the  other  will  affect 
the  places  of  both  of  these  stars.  But  it  will  affect  the 


Apparent  Places  of  the  Stars  179 

nearer  of  the  two  stars,  let  us  say,  ten  times  as  much  as  it 
will  the  more  remote.  In  fact  we  may  for  the  present 
consider  that  the  remoter  star  has  not  moved  at  all  and 
that  the  relative  displacement  has  to  be  entirely  attributed 
to  the  nearer  of  the  two  stars.  The  astronomer  will 
therefore  carefully  measure  the  angle  between  the  two  stars 
which  we  have  supposed  to  be  visible  in  the  same  field  of 
view  in  his  telescope.  This  is  one  of  the  astronomical 
measurements  which  can  be  conducted  with  a  very  high 
degree  of  accuracy.  We  shall  suppose  that  the  measure- 
ments are  repeated  six  months  later,  by  which  time  the 
astronomer  has  been  carried  to  the  opposite  diameter  of 
the  orbit  of  the  Earth. 

In  the  adjoining  figure  (Fig.  24)  the  remoter  star  is 
supposed  to-  be  so  far  off  that  the  lines, 
AS2  and  BS2,  drawn  to  it  from  two  positions 
of  the  Earth  at  opposite  sides  of  its  orbit  are 
indistinguishable  from  a  pair  of  parallel  lines. 
The  nearer  star,  whose  distance  we  want  to 
measure,  is  represented  at  S^  Then  when 
the  Earth  is  at  A  we  measure  the  small 
angle  SiAS^  and  when  at  B,  six  months 
later,  we  measure  the  small  angle  8^8%. 
Now  since  AS2  and  BS2  are  supposed  to  be 
sensibly  parallel  it  follows  that  the  angle 
AOB  =  SiBS*  But  the  angle  p.  24 

AS,B  =  AOB  -  S.ASt, 

wherefore  AS^B  =  S}BS2  -  SiAJSg.  Thus  we  see  that  the 
angle AStB  is  determined. 

But  this  angle  is  the  angle  subtended  at  the  star  by 
the  diameter  of  the  Earth's  orbit,  and  it  is  a  simple 
calculation  to  find  out  how  far  off  the  star  must  be  placed 
in  order  that  a  line  186  millions  of  miles  long  should 
subtend  an  angle  at  it  equal  to  that  value  which  we  find 
for  AS,B. 


180 

For  example,  if  we  found  that  on  the  21st  June 
6V4S2  =  45",  and  on  the  21st  Dec.  that  S^S*  =  47",  then 
of  course  ASiB  =  2".  But  it  can  be  easily  shewn  that  in 
an  isosceles  triangle  whose  vertical  angle  is  2"  the  sides 
are  103,132  times  as  long  as  the  base.  Therefore  the  star 
in  this  case  must  be  103,132  x  186,000,000  miles,  or  more 
than  19  billions  of  miles  away. 

Half  the  angle  ASiB,  or  the  angle  subtended  at  the 
star  by  the  radius  of  the  Earth's  orbit,  is  called  the  star's 
annual  parallax. 

Of  course  the  triangle  AS^B  will  not  generally  be 
isosceles,  as  we  have  assumed  for  convenience  of  explana- 
tion, but  calculation  will  enable  the  parallax  to  be  found 
in  other  cases. 

It  may  be  remarked  that  no  star  has  been  found  to 
have  a  parallax  as  large  as  1",  as  we  have  supposed,  from 
which  we  conclude  that  no  star  exists  within  a  distance  of 
19  billions  of  miles  of  the  Sun.  The  nearest  star  yet  found 
is  situated  at  a  distance  of  25  billions  of  miles. 

The  great  majority  of  the  objects  which  we  see  each 
night  are  at  distances  so  great  that  they  have  not  been 
determined.  In  most  cases  indeed  there  is  little  hope  of 
determining  them.  This  consideration  gives  us  an  im- 
pressive notion  of  the  scale  of  the  Sidereal  System. 


INDEX 


(The  numbers  refer  to  the  pages.) 


Aberration  of  light,  172 

Adams,  144 

Alcyone,  NiO 

Andromeda,  great  nebula  in,  103 

Annual  equation,  95 

Annual  motion  of  Sun,  47 

Annular  eclipse,  67 

Arctic  day  and  night,  51 

Arcturus,  7 

Argelander's  chart  of  stars,  8 

Asteroids,  112 

Atlas,  161 

Atmospheric  refraction,  5 

Barnard,  127,  130,  148,  160 

Belopolski,  102 

Belts  on  Jupiter,  121 

Binary  stars,  169 

Bode's  law,  143 

Bond, 138 

Bouvard,  142 

Bradley,  173 

Bright  emission  lines,  38 

Canals  on  Mars,  109 
Oapella,  7 
( 'arbon  in  Snn. 44 
Cassini's  line  on  Saturn,  131 


Celaeno,  161 

Centaur,  cluster  in,  159 

Challis,  144 

Chemical  elements  in  Sun,  40 

Chronograph, 17 

Circles  of  the  Sphere,  11 

Clouds  on  Jupiter,  122 

Colours  of  double  stars,  169 

Comets,  147 

Constellations,  7 

Copernicus,  4 

Corona,  42 

Crape-ring  on  Saturn,  131 

Dark  lines  in  spectrum,  35 
Day,  seasonal  change  in,  51 
Day,  sidereal,  12 
Declination,  14 

Diameter  (apparent)  of  Sun,  54 
Dispersion  of  light,  .'50 
Diurnal  motion,  9 
Double  stars,  166 
Dumb-bell  nebula,  163 

Earth,  rotation  of,  13 
Earth,  size  of,  5 
Eclipses  of  Moon,  6.'! 
Eclipse  of  Sun,  41 


181 


182 


Ecliptic,  47 

Ecliptic,  obliquity  of,  49 

Electra,  161 

Ellipse,  motion  in  an,  83 

Elliptic  movement  (apparent)  of 
Sun, 54 

Equator,  11 

Equatorial  protuberance  of  Ju- 
piter, 120 

Equinox,  18 

Equinoxes,  precession  of,  97 

'Eros,  104,  114 

Fraunhofer  lines,  34 

Galileo,  127 
Galle,  144 
Gravitation,  82 

Hall,  Professor  Asaph,  111 
Halley's  Comet,  148 
Heat  emitted  by  Sun,  45 
Henri,  the  Brothers,  101 
Hercules,  cluster  in,  159 
Herschel,  Sir  John,  159 
Herschel,  William,  104, 138,  140 
Holmes'  Comet,  164 
Horizon,  12 
Huggins,  136 

Invisible  light,  33 
Iron  lines  in  Sun,  37 

Jupiter,  118 

Jupiter,  weight  of,  128 

Keeler,  136 
Kepler's  laws,  82 
Kirchhoff ,  39 

Lagrange,  91 

Langley,  on  Sun's  heat,  45 


Laplace,  91 
Lassell,  138 
Latitude,  21 
Leonids,  153 
Le  Verrier,  144 
Libration  of  Moon,  G8 
Lowell's  theory  of  Mars,  110 
Lunar  craters,  70 

Magnitudes  of  stars,  7 

Maia,  161 

Mars,  104 

Mass  of  planet  determined,  92 

Maxwell,  135 

Mercury,  98 

Meridian,  12 

Meridian  circle,  16 

Merope,  161 

Milky  Way,  160 

Milky  Way,  structure  of,  8 

Moon  in  eclipse,  65 

Moon's  motion,  60 

Moon,  origin  of,  81 

Moon,  size  and  weight,  62 

Moon's  surface,  features  of,  72 

Motion,  laws  of,  86 

Multiple  stars,  170 

Nadir,  12 

Neptune  discovered,  142 

Newton,  85 

Gibers,  114 

Orion,  great  nebula  in,  163 

Parabolic  orbit  of  comets,  147 

Parallax,  annual,  180 

Path  of  Sun,  54 

Periodic  changes  in  Sun-spots, ' 

Perrotin,  109 

Perseus,  cluster  in,  159 

Perturbations  of  Moon,  94 


Index 


183 


Perturbations,  planetary,  88 

Phases  of  Moon ,  61 

Photography  in  spectroscopy,  32 

Photosphere,  42 

Pickering,  W.  H.,  138 

Pickering,  P  'ofessor  E.  C.,  163 

Planetary  perturbations,  88 

Pleiades,  16C 

Pointers,  10 

Polar  snows  on  Mars,  106 

Pole  Star,  10 

Precession  of  equinoxes,  97,  173 

Prism,  30 

Proper  motion  of  stars,  157 

Refraction,  5 

Refraction,  effect  on  sunrise,  55 

Right  Ascension,  14 

Rings  of  Saturn,  131 

Roche,  135 

Rotation  of  Earth,  13 

Rotation  of  Sun,  27 

Rotation  of  Venus,  101 

Satellites  of  Jupiter,  125 

Satellites  of  Mars,  111 

Saturn,  130 

Schiaparelli,  102 

Secular  acceleration  of  Moon,  96 

Shooting  stars,  152 

Sodium,  lines  of,  40 

Solar  day,  56 

Solar  spectrum,  35 

Spectroscope,  29 

Spectroscopic  binaries,  170 


Sphere,  the  celestial,  7 

Sphere,  circles  of  the,  11 

Stability  of  Saturn's  rings,  133 

Stokes,  Sir  G.,  39 

Sun,  light  and  heat,  25 

Sun,  mass  and  density,  25 

Sun,  size  of,  24 

Sunlight,  composition  of,  29 

Sun-spots,  25 

Swift's  comet,  148 

Synodic  period  of  Moon,  62 

Taurus,  rising  and  setting,  2 

Taygeta,  161 

Tidal  evolution,  77 

Tides,  75 

Transit  instrument,  16 

Twilight,  57 

Tycho  Brahe,  95 

Uranus,  140 

Ursa  Major,  movements  of,  9 

Vega,  7 

Venus,  99 

Vernal  equinox,  49 

Vesta,  114 

Volcanoes  in  Moon,  73 

Wilson,  W.  E.,  159,  163 
Witt,  115 

Zenith,  12 
Zodiac,  4 
Zodiac,  signs  of,  48 


ELEMENTS   OF   PHYSICS 

FOR   USE   IN   HIGH   SCHOOLS 
By  HENRY   CREW,  Ph.D. 

Professor  of  Physics  in  Northwestern   University 

I2mo.     Cloth,    xiv  -f  347  pp.     Price,  $1.10 

The  treatment  differs  from  other  elementary  books  on  the  same  sub- 
ject in  that  it  is  more  consecutive.  The  aim  has  been  to  build  upon 
the  average  experience  of  a  student,  and  to  unify  the  discussions  of 
Mechanics,  Sound,  Heat,  Light,  and  Electricity  in  such  a  way  that 
even  the  beginner  does  not  feel,  in  passing  from  one  to  the  other, 
that  he  is  undertaking  a  totally  new  study.  By  this  plan  it  is  hoped 
that  the  high  school  student  will  obtain  the  soundest  and  most  eco- 
nomical training,  whether  for  the  sake  of  liberal  culture  or  for  later 
use  in  college  work,  engineering,  or  medicine.  The  treatment  is  at 
every  point  experimental  and  quantitative. 

TABLE  OF  CONTENTS 

INTRODUCTORY.  Chapter  I.  —  Motion.  Chapter  II.  —  Simple  Har- 
monic Motion.  Chapter  III.  —  General  Properties  of  Matter.  Chap- 
ter IV.  —  Special  Properties  of  Matter.  Chapter  V.  —  Waves.  Chap- 
ter VI.  —  Sound.  Chapter  VII.  —  Heat.  Chapter  VIII.  —  Magnetism. 
Chapter  IX.  —  Electrostatics.  Chapter  X.  —  Electric  Currents.  Chap- 
ter XI.  —  Light.  Appendix  to  Chapter  IV. 


COMMENT 

"It  seems  to  me  that  heretofore  new  text-books  on  elementary  physics 
and  new  editions  of  old  ones  (with  some  few  exceptions),  have  been  new 
merely  in  that  they  appeared  in  new  covers  and  had  been  filled  out  a  little 
by  the  incorporation  of  a  few  new  and  remarkable  discoveries.  Professor 
Crew  has  written  a  new  book  from  beginning  to  end,  and  I  doubt  if  his 
method  of  treating  the  subject  could  be  improved  upon." 

—  PROF.  R.  W.  WOOD,  University  of  Wisconsin. 


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•     66   FIFTH   AVENUE,    NEW   YORK 


OUTLINES   OF   PHYSICS 

AN   ELEMENTARY  TEXT-BOOK 
By  EDWARD    L.    NICHOLS 

Professor  of  Physics  in  Cornell  University 

121110.     Cloth,     xi  +  452  pp.     Price,  $1.40 
Questions  to  same,  price  10  cents 

In  this  volume  the  author  has  outlined  a  short  course  in  physics 
which  should  be  a  fair  equivalent  for  the  year  of  advanced  mathe- 
matics now  required  for  entrance  to  many  colleges.  The  subject  is 
divided  into  five  parts  as  follows :  — 

PART     I. -Mechanics 

PART    II.  — Heat 

PART  III.  — Electricity  and  Magnetism 

PART  IV. -Sound 

PART    V.- Light 

Appendices 

A  combined  class-book  and  laboratory  manual  which  is  logical  in 
arrangement  and  clear  in  its  statement  of  principles  and  descriptions 
of  experiments. 

COMMENTS 

"  Nichols's  '  Outlines  of  Physics '  is  the  first  satisfactory  elementary 
physics  I  have  ever  seen,  after  searching  seven  years  for  one.  We  shall 
use  it  next  year." 

—  PROF.  JAMES  BYRNIE  SHAW,  Illinois  College,  Jacksonville,  111. 

"I  note  extreme  clearness  and  simplicity  of  explanation  in  the  text;  all 
useless  details  are  omitted  and  the  author  aims  at  his  point  at  once,  so  that 
one  cannot  help  reading  ideas  instead  of  words.  Another  plan,  which 
seems  to  me  to  be  an  excellent  one,  is  the  placing  of  the  descriptive  text 
before  the  experiment  to  be  performed,  so  that  the  experiments  serve  to 
verify  the  author's  statements.  .  .  .  Good  judgment  shown  in  selecting 
simple  apparatus  for  performing  the  experiments.  As  an  all-round  up-to- 
date  book  it  is  the  best  I  have  ever  seen." 

—  R.  WESLEY  BURN  HAM,  High  School,  Gloucester,  Mass. 


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Elementary  Lessons  in  Electricity 
and  Magnetism 

By  Professor  SILVANUS    THOMPSON 

[First  Edition,  1881 ;    reprinted  1882  (2),  1883,  1884,  1885,  1886,  1887,  1889, 

1890  (2),  1891  (2),  1892  (3),  1894.     Second  Edition,  January, 

1895;  reprinted  November,  1895,  1897,  1899.] 

NEW   EDITION    REVISED    THROUGHOUT   WITH    ADDITIONS 
8vo.    Cloth,    xv  +  634  pp.    Price,  $1.40 


"  From  beginning  to  end  the  subjects  are  judiciously  chosen,  admirably 
dealt  with,  and  logically  arranged,  forming  as  a  whole  what  is  unquestion- 
ably the  standard  elementary  text-book  of  the  day.  We  do  not  say  it  is  the 
best ;  we  go  further,  and  say  it  is  the  only  book  we  can  honestly  recom- 
mend to  the  junior  student." 

NATURE  —  "Whoso  seeks  a  class-book  on  electricity  and  magnetism, 
containing  an  elementary  exposition  of  recent  work,  will  find  their  want 
supplied  by  Professor  Thompson's  lessons." 

A    PARTIAL   LIST  OF  ADOPTIONS 

University  of  California.  Commercial  School,  Buffalo,  N.Y. 

Washington,  D.C.  Board  of  Education,  New  York  City. 

Athens,  Ga.  Horace  Mann  School,  New  York  City. 

University  of  Illinois.  Y.  M.  C.  A.,  New  York  City. 
Rose  Polytechnic  Institute,  Terre  Haute,       Rochester,  N.Y. 

Ind.  Utica,  N.Y. 

University  of  Indiana.  Clarkson    Memorial    School,    Potsdam, 
Purdue  University.  N.Y. 

Iowa  City,  la.  Rensselaer  Polytechnic  Institute,  Troy, 
University  of  Kansas,  Lawrence.  N.Y. 

Baldwin,  Kas.  Trinity  College,  Durham,  N.C. 

Center  College,  Danville,  Ky.  Raleigh,  N.C. 

Lexington,  Ky.  Ohio  Wesleyan  University,  Delaware,  O. 

Baltimore,  Md.  Pennsylvania  Military  Academy,  Ches- 
Harvard  College,  Cambridge,  Mass.  ter,  Pa. 

University  of  Michigan.  Temple  College,  Philadelphia. 

Rolla,  Mo.  Erie,  Pa. 

Stevens  School,  Hoboken,  N.J.  Pittsburg,  Pa. 

Y.  M.  C.  A.,  Brooklyn,  N.Y.  Clemson  College,  S.C. 
ManualTrainingHigh  School,  Brooklyn.       Clarksville,  Tenn. 

Boys'  High  School,  Brooklyn.  University  of  West  Virginia. 

Pratt  Institute,  Brooklyn.  Y.  M.  C.  A.,  Richmond,  Va. 


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The   Elements   of   Alternating 
Currents 

BY 

W.  S.  FRANKLIN  and  R.  B.  WILLIAMSON 
8vo.    Cloth.     205  pp.     Price,  $1.75 

This  book  represents  the  experience  of  seven  years'  teaching  of  alter- 
nating currents,  and  almost  every  chapter  has  been  subjected  repeatedly 
to  the  test  of  class-room  use.  The  authors  have  endeavored  to  include 
in  the  text  only  those  things  which  contribute  to  the  fundamental 
understanding  of  the  subject  and  those  things  which  are  of  impor- 
tance in  the  engineering  practice  of  to-day. 


CONTENTS 

CHAPTER  I.  —  Magnetic  flux.  Induced  electromotive  force.  Induc- 
tance. Capacity. 

CHAPTER  II.  —  The  simple  alternator.  Alternating  e.m.f.  and  current. 
The  contact  maker. 

CHAPTER  III.  —  Measurements  in  alternating  currents.  Ammeters. 
Voltmeters.  Wattmeters. 

CHAPTER  IV.  —  Harmonic  electromotive  force  and  current. 

CHAPTER  V.  —  Problem  of  the  inductive  circuit.  Problem  of  the  in- 
ductive circuit  containing  a  condenser.  Electrical  resonance. 

CHAPTER  VI.  —  The  use  of  complex  quantity. 

CHAPTER  VII.  — The  problem  of  coils  in  series.  The  problem  of  coils 
in  parallel.  The  problem  of  the  transformer  without  iron. 

CHAFFER  VIII.  —  Polyphase  alternators.     Polyphase  systems. 

CHAPTER  IX.  —  The  theory  of  the  alternator.     Alternator  designing. 

CHAFFER  X.  —  The  theory  of  the  transformer. 

CHAPTER  XI.  —  Transformer  losses  and  efficiency.  Transformer  con- 
nections. Transformer  designing. 

CHAFFER  XII.  —  The  synchronous  motor. 

CHAPTER  XIII. — The  rotary  converter. 

CHAPTER  XIV.— The  induction  motor. 

CHAPTER  XV.  —  Transmission  lines. 


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